Question on Rear end Posi/Eaton/Locker

[Senor_paco]

I have a 77 silverado 1/2 ton, with 3.07 gears. Am i correct to assume I have a 7 5/8 or an 8 3/4 rear gears? The reason I'm asking is I want to get a locker or eaton unit for it, and want to make sure I get the right one. I'll problably go up on the gears to a 3.42 or 3.73. It is just a weekend driver for grins, so fuel economy doesn't matter, even though I'll get a 700r4 later. any help is appreciated!! I'm tired of the one legged animal!