I assumed that the 2.6 PU is a 4WD, if it's a 2WD, there's a lot of different drivetrains you could stuff into there including a SBC V8. In that case I wouldn't mess around with a 4.3, as you're doing the same amount of work as you would for a V8.
A nice later model Chevy Vortec 4800 (275 HP if I recall !) would be a nice motor for a small truck. That should let it get out of its own way !!!
Now, if you have a 4WD that's different because having a transfer case to deal with makes things complicated.
You might consider sticking with a somewhat 'stock' setup by finding a donor V6 vehicle. Any '89-'91 Trooper with GM 2.8 V6 or an early 4WD Rodeo / P'up with the GM 3.1 V6 would be a likely donor. Find a wrecked or rusted-out rig and strip all the parts you'd need. A lot less difficult than trying to design your own swap and doing all your own R&D.
BTW a GM 3.4 out of a Camaro/Firebird or new GM HT crate motor, has as much HP as a stock 4.3 (160) but a bit less torque. A very good performer in a Trooper and probably much better in a truck as your P'up weighs less.
You can build a 3.4 using mostly bolt-ons and get close to 190-200HP. Plus the 60 Deg V6 is smoother and lighter than a 4.3.
Frankly, I wish they'd have built a V6 Spacecab, but they never did. One with a 3.4 would be a Sweet Ride indeed and would get reasonable mpg, too.
I have files for the 2.8-for-3.4 swap, which of course would not be all you need for a 4-cyl-to-6-cyl conversion, but it would at least be a start.
If you're interested, send me a P.M. with your e-mail address and I'll forward the info to you.
Happy Swapping............ed