They only made 10 bolt and 14 bolt rear ends for this body style truck. I know a truck salvage yard in San Diego, Ca that has a 14 bolt. The newer body style trucks are 6 lug like the 4x4. You could also look for a 12 bolt from a square body. it might fit. A lot of them came with a posi and...
I prefer the 73 to 78 body style. But they want too much for just a decent truck. I had a 78 4x2 long bed that got stolen. I replaced it with a 92 short bed extenda cab for the same money as my 78. I prefer regular cab too, but no one is willing to sell for a fair price.
But I still have my 78...
The tuned port system from the F-body cars is good for low end power. If you look back into some of the super Chevy or hotrod magazines, the did some testing on the tune port engines and said they are a better fit for a truck than a car.
For A 4X4 it might be better to go with a centrifugal supercharger like a Vortech unit or a pro charger. Not as efficient as a turbo, but you will not have to replace as many parts and you should pull a little more bottom end power.
Just recommending another option that might be a little...
I know I am a little late. You could try a remote turbo. Place the turbo farther back in the exhaust system so you keep heat out of the engine compartment and can run what ever headers you want.