Suggestions for upgrades to tow? 79 C10

[merman]

Hi all, I've browsed around and seen a variety of threads and suggestions on rear end and engine/trans combos for towing. I have a 79 longbed RWD that I'm looking to upgrade for towing. It currently has a 350 in it, but I'm not sure about the trans or rear end; I'm planning on replacing the trans and rear end to suit my needs. If anyone has any suggestions for towing 6000lb, I would appreciate the input. Also, I do plan on going up mountains fairly regularly with it. Thanks in advance!