[dsraven]

lots of guys use the big trucks as donors for their cabs because the cabs are usually in pretty good shape since the big trucks don't see as much use as the smaller trucks. they then get the front sheet metal for the smaller truck and assemble the unit onto a smaller truck frame. just a thought.
if you want a big fendered unit thats great too, just find a modern truck with the capacity you want and put the old truck cab and front end onto that set up. you might look at auctions or insurance lots for a donor. a donor is a great way to go because it comes with just about everything you need to do the swap.
for more info on frame drawings etc look at the gmupfitters site as they have a lot of info there under body builder manuals, including frame drawings with dimensions. i would think a modern medium duty truck witha diesel engine and the trans type of your choice wouldn't be a far stretch to think your old cab and front end could be grafted onto it.