68 rear change

[67 chevelle]

I have a 68 c10 project truck , its got " one wheel peel " , and 273 gears , I have to upgrade , its been lowered 3-5 , got discs in front , and is getting an ly6 and turbo 350 , whats the best , easiest , cheapest , way , id like to keep the 6 on 5.5 , so gears and a posi are my asumption , who to get them from , truck pinions are longer i think so i cant change to car gears , ive got an 69 chevelle 12 bolt with 331 gears but no psi , i dont think they will interchange , any help would be apreciated