I think the springs would be fine with a 350, but they will give a stiff ride and it will sit too high in the rear if you're going MII in the front. You'll probably want to change them out or possibly remove some leaves or de-arch.
I think a one piece drive shaft will be a little long on that truck. A lot of differing opinions on this, but I think your maximum length is going to want to be in the 50" to 55" range from what I've been told or read in the rodding magazines.
