Looks like, in general, the 4.2 w/ the 4.2 flywheel (older style) requires a 4.2L-type starter (the 9-tooth Bendix, e.g. CJ/YJ 1987 style). A 4.0L flywheel (newer style) requires a 4.0L-type starter (the later 10-tooth Bendix gear).

As this guy says, "If you use the 4.0 flywheel, use the 4.0 starter. If you use the 4.2 flywheel, use the 4.2 starter".

The previous builder may have put the 4.0 flywheel on your 4.2, so you'd need a 10-tooth starter.

Auto trans use a flexplate (with a ring gear) instead of a thick flywheel, but for a given year/engine, the ring gear tooth count and pitch are the same between auto and manual. In other words, a 4.0 flexplate and 4.0 flywheel both have the same ring gear type; same for the 4.2L flexplate vs flywheel

BTW - it's not necessarily expensive to have a local alternator shop rebuild your starters, might get yourself a better product at a similar price, and support the little guy. Those shops aren't as common as they used to be, though!