R&D-
I can't profess to being an expert here, but that won't stop me fom sharing my opinion! I think the old rear is possible, but keeping it will present new challenges. Depending on the manual transmission you use (are you sticking with granny gear or looking for something that went with a late model 350?), there may be real problems in connecting power to the (stock?) 48 driveshaft (in a torque tube, right?) and rear differential. Keeping the stock shaft-differential set-up may be more expensive than using the donor rear and purchasing new wheels. I would consider the gear ratio in the 48 rearend as something you would prefer to change if this beast will be doing any highway travelling. Also, the donor rear will likely have better suspension, so the ride could be enhanced dramatically. For simplicity and less hassle, I would recommend using the donor rearend from a compatible donor. In my own experience, we just needed to find a junk driveshaft of the right length to make the switch and those are not very expensive. You'll spend money to get it in place, but you'll gain in power transfer, lack of vibration, better ratios, and a smoother ride. You may be able to resell the 8 lug wheels too.
Just my 2 cents-
