Welcome to The Bolt --- And WOW...what a great looking find. Locating anything that vintage with what appears to be minimal putzing is quite an accomplishment these days.
As for wheels/tires/mph issues, there are only so many choices and they are related. The mongo 20 x 5 wheels and rims look great but the only tires available are Chinese, bias ply tube type. And the gearing is indeed only capable of 45-50 tops. Fine for a pre-Interstate highway load hauler or a parade vehicle, but not that practical for general service these days.
You can pretty much forget finding any higher gearing for that rear end. There are "stories" about a slightly higher 5.17 ring & pinion set, but in over three years of searching, I never spoke with anyone who had ever seen such a thing. That leaves going to a later model rear axle, of which there are several that will fit nicely, but none will take the original rims without a fair amount of work. I went with a '98 Chevy 3500HD Dana 80 dually rear axle for my '46 1.5T as it does have the same lug pattern but takes a 19.5 x 6 tubeless rim. Not quite as mongo as the factory mounts, but close. And the tire choices are much better (modern tubeless radials). As for that mph thing...the Dana has many choices that when paired with the right tranny will get you up to speed (60-80 depending on the combination of axle ratio, final drive ratio and tire diameter). There are some great online calculators that make it easy to test out various combinations. A link to one is below.
Then there is the matter of stopping that big beast after you get it going faster than it was ever intended to go. There again, updating is an option, but one worth considering for anything but a museum piece. Properly maintained, all the original gear is fine, but it does have its' design limits. Just decide what you want to do with that darlin' then build accordingly.
The chopped frame is a simple welders' fix unless you are going for show points. In fact...I have a 1942 long wheelbase 2-ton frame (195" ?) that you can have for the taking. I'd much rather contribute it to any working project than scrap it and I think it would fit.
Whatever you decide, keep us updated...we love pix of other people working.
Gear/speed Calculator
http://www.csgnetwork.com/multirpmcalc.html 
