Which Engine?

[Grumpyroger]

I need some input from you Guru's out there! Let me start by saying Thank You in advance for any help you might offer.

I have a 1969 C-20, I acquired from a Gentleman down the street about 10 years ago, it was in really rough condition, and I am still working on it. It came with a 350 CI engine, TH400 trans and 4.10 gears. Seeing that the truck is 43 years old and still runs good (I tow a 23' travel trailer with it) I know it will be due soon for an engine rebuild. (Trans done last year) I want to set it up for a good amount of Torque, should I just built a Stroker 383 or should I change it out for a BB 454 or something.
I have remove all of the smog (not required) and I want to replace the TH400 with a 700R4 Trans. What do you think?

I also have problems towing the trailer up hills, and that's the reason I need more power and torque. Any Ideas?