Looking to Add a LSD/Posi to my Classic Car…

I’ve got a 1986 Chevrolet Caprice Brougham that I’m restoring. The differential is leaking badly and the shop told me that they would install a LSD/Posi for free given I provide the part.

The problem is that I’m having trouble finding a LSD/Posi for my gear ratio which is a 2.56. Aside from this I plan on leaving the car as original as possible, so maybe I’m searching in the wrong places? I haven’t had any luck finding it and I really don’t want to go up to a 3.xx gear ratio, but that may be my only option. Should I keep looking or suck it up and change the gear ratio?

If anyone knows anywhere I should check, do let me know. My differential is a 7.5" 10 Bolt 26 Spline w/ 2.56 gear ratio