Both of my cars have 164 tooth flexplates/bellhousings, etc, on their C4's. Note that the flexplate/converter bolt pattern is different between the two, and there are probably small bolt circle large converters, and vice versa.... Measure before you buy....Been there done that. A 10 in converter should fit fine in a 157 tooth bellhousing, as I think stock was 11 in or so.
I have a 10 in TCI in the 82 that works fine. It's about a 2800-3000 stall. Normally you don't use a lockup converter with a C4, that would be from a C5, which is another set of matchup deals.
Depending on your engine combo and planned rpm, an higher stall 10 in or even a 9 in would work. The 9 in is usually a race only 3500 and up stall unit.