For goodness sakes, why change the frame? Why is this the first thing people think about, how about make the one that is there work to your liking?
There is NO SUCH THING as a frame "swap". It is simply swapping time, money and work from one frame to another.
You have worked so hard to get that truck back, keep it THAT truck and not an S-10 with that trucks body parts stuck on it.
What are your expectations? Do you want to get out on the highway and take it on 5000 mile trips? Maybe upgrading the suspension is the way, maybe an S-10 frame is the way. But what do you WANT out of the truck.
If you want a neat old hot rod, put a dropped axle or a Mustang II front end, there are ZILLIONS of kits out here. There are ZILLIONS of kits out there for the rear, super nice springs and shocks that will make it ride like a dream.
I have to tell you I have driven these trucks (have had one for 40 years) and with a proper rebuilt suspension and a dropped axle they drive VERY nice.