Thanks for all of the information, I think that adapting a toyota trans is becoming the most attractive option, if I can get one on there that has either a 4th (auto) or a fifth (manual) at or around .83 or taller, my math says I should be around 74.26 mph @ 2850rpm and that is doable. Yes one of the qualifications for any fix applied to this truck is that it has to be cheap, I dont mind spending money on it, but if you drop 300 here and 500 there before you know it you have 6 or 7k in building something that you could have bought (dodge cummins) for around 8 or 9k , it defeats the purpose.
Anyone have any experience making adaptor plates, I was thinking a 1/4 sheet of steel. I could cast an aluminum one, but I cant machine anything that large on my lathe.
Im not sure I agree about the power, while the larger that a 300d tires will require more power to move, it is also geared lower which will have the opposite effect of the tires, also the relevant specs on the MB 300d are:
3582 lbs / 123hp / 181# torque

and the niesel
3600 lbs / 103hp /134# torque

so these vehicles are comparable, but you are right the larger tires will require a little more power, but the extra torque (35% more) should offset it a little, I know I wont be breaking any land speed records in it, but I think it will get up to speed