I am wondering the same thing. I know its possible to put my 5-speed and stock bottom end T1 together, but if I'm going to get cooling problems, it's going to make me think twice. But, where there's a will, there's a way.
Oh, and the 901 box was used with a 165-15 tire, or a 185-70-15 radial option. My dad uses a 195-65-15 tire on his 912 (4 speed, but same final drive), and that is close to a 205-65, which is close to a 165 (stock beetle). Whew, that was a mouthful
I'd say that if you wanted a smaller tire, go with a 205-60-15 or a 205-45-17.
Also, if your engine is stock, or close to it, you may not have to worry as much about overheating as if you had a bigger engine. What I mean is that it may take longer to reach that critical temperature. Well, once someone with experience in this particular area chimes in, we should know.
OH! Also keep in mind that the 901 box was used in the 912! Duh! Well, that means that it worked with a 90hp 4-cyl engine as well as it did with a 100+ hp 6-cyl. I dont think we have much of anything to worry about.