Here is what I know about turning your standard Bus into a Baja Bus. Before you go any further take this as my disclaimer. I'm no mechanic and haven't done all of these modifications myself yet, although I've investigated all of them and spent many hours talking to guys who have with the full intent of doing them myself, I'm just working my way down the list and through the cash at the moment.
What this means is double check everything before you cut, bend, beat or modify anything and then double check again. These steps are really guidelines but they are as accurate as I can make them, so if anything goes wrong don't say I didn't warn you.
After all that lets get on with it, most of this stuff is pretty easy, so get to work on your Baja Bus.
I'll start with the front wheels and tires as they're probably the easiest, mainly because you've got less options.
Without any lift at all you can fit a 205/75/15 in the guards, this is what I run on my Bus and any bigger would probably scrape somewhere with full steering lock.
If you lift the Bus more than about four inches you can fit more tire under there, even then a 215/75/15 is about it though. Any larger and you'll need to restrict the distance the suspension compresses otherwise the tire will hit the inner guards due to its height.
Check out the front end lift stuff to see how that's done. Remember though, too big and the steering becomes heavy and you just have problems.
If you're after a cheap 15" wheel that fits the front, try and hunt down a Mercedes wheel with the codes ET37 6.5x15h2. This wheel will provide the right offset for the front and will take a 205/75 with no problems.
Ok, that was pretty easy, now onto the back.
There's a heap more options here, it's just a matter of how far you want to go.
Anything up to a 33"x12.5" on a 15" wheel will fit but will take some serious modification.
A much easier fit is a 235/75/15, even this is about as much as you can get without cutting the guards. It's about the same size as a 29"x9.5". Height is not so much a problem on the back, width is where you get stuck.
A 235mm or 9.5" is about all you can fit, you may squeeze a 10.5" under there but I don't think so, not without a serious lift.
If you want to go taller a 30"x9.5" will fit and a 33"x9.5" should fit but you may need to cut the inner guards and beat the crap out of them with a hammer where the tires rub on suspension compression. Tall tires will have a big affect on the gearing as well so you need to take this into account.
It's easy to cut the guards if you want to fit a wider tire. Stick your head under the rear guards with the wheels off, there's a noticeable seam, this is where you'll need to cut. Once it's cut the rear wheel opening should be about the same height as the front. Be careful when you cut though, the metals thin and if you use heat it might distort. Your best bet is to use a sheet metal cutter or drill lots of holes and then tidy it up with a grinder.
Once you've done the cut stick on a rubber flare from an RV shop and your done, now it's just a matter of bolting on those fat wheels!!!
There's another Mercedes wheel for the back, ET25 7X15h2, it might just fit without cutting if you run a 235/75/15, any wider and you'll have to cut with this wheel or go for the serious lift.
After all this tire and guard work you need to be very careful the first time you
open the sliding door, if the wheel offset isn't correct or the tires are very wide the
door can end up with as nice bend in it as it hits the tire when you slide it back!!!
I did see a nice custom bus with flared guards, one had a big gouge in it where the
guy building it forgot about this little problem, it was not a nice look!!!
If you lengthen the arm by an inch then move the stop towards the back of the bus
If you've got a 2.0lt bus your lucky, you should have the strongest transaxle available in the early Buses, but strength isn't the only thing to consider here.
Before you go stuffing the tallest tires you can under the back of the Kombi you need to consider the effect they'll have on the overall gearing. The taller the tire the taller the gearing, what this means is the road speed will increase with the increase in wheel size without the rev's increasing.
This is really important if your running a standard 2.0lt Bus and you want to put tall tires on. With the taller gearing of the 091, thats a 2.0lt transaxle, you'll have problems turning 33" tires when your pulling away in 1st.
You'll need to slip the clutch more and give it some more revs, you might be able to live with this on the street, although I doubt it, but in the sand you won't have the torque to get moving. You need to remember that a 33" tire is about 8" taller than standard.
Below is a chart showing the revs you'll get at 10kmh (6.2mph) in 1st gear with different tire sizes running a standard 091 transaxle compared to a 1600 transaxle.
As you can see if you put 33" tires in a Bus running a 2.0lt transaxle you'll end up doing 1091rpm at 10kmh compared to the standard 1423rpm with normal tires, but if you swap to a 1600 transaxle you'll bring the revs back up to 1290rpm at 10kmh which is much better although still quite low.
The 29" tires would work better giving you 1468rpm at 10kmh compared to the standard 1423rpm with a 2.0lt transaxle. Maybe a 30" or 31" tire would be better still, maybe you could let me know ;)
I've also included mine, as you can see it only does 929rpm at 10kmh in 1st!!! Thats very low but I'm running a 270cid V8 engine which puts out 210nm of torque at these revs, this is very important as it's the turning force the engine can apply to the wheels. A standard 2.0lt engine puts out 143nm of torque at 2800revs!! At 2200revs I'm puting out about 330nm.
If you have a 1600 Bus forget about puting tall tires on, you just won't be able to turn them, check out Sandlizrds tire details, he had trouble with 31" tires in a Bug which, as you know, is much lighter. The lower bus gearing will help but there just isn't the power needed, you could go with a 28" tire but don't go too wide or the mass of the tire will work against you as well. This setup will be OK as long as you don't want to go anywhere too extreme.
What this means is if you have a lot of torque then go with whatever transaxle you want, or even make the gearing taller to improve cruising on the freeway. If your running a standard 2.0lt engine consider changing to a 1600 transaxle to help bring the gearing back to normal with the tall tires and if your running a tuff 2.0lt engine then stick with the 2.0lt transaxle but be careful with tire sizes to make sure you don't go too tall.
If you want to try other tire sizes and gear ratios check out Automotive programs by Bowling and go to the top frame and look for Bowling's RPM and MPH calculator or better still download my Speed and RPM program and print of graphs and see all the stuff you need to know about speeds and RPM in real detail.