I have a truck rear end that was narrowed. Now i need help acquiring the center section, prefer a posi unit. The pumpkin is open to the front. What unit is the best to seek when searching used?
part numbers to look for?

If it's a Mopar 8.75 housing you have, lot of info on it at this site.

A "742" sure grip with the ratio you want. Those are the last 3 of the large cast in numbers on the side of the case. Might put an ad in your local CL & also see what's for sale here on Moparts.

Which gear ratio would you be looking for ??

they can all handle quite a bit of HP. 742 and 489 have a better selection of gear ratios. 741 742 489 have different pinion stem diameters, but this is not usually where they fail. so why are some better than others?

I have a 727 auto. Probably run gears something good with gas milage but some get up and go. I measured the shaft somewhere in the middle and got 1.50"??

Good mileage 3.23. A little more get up and go 3.55.

If you really want a "posi" you need a GM rear end to start with..