Do you know what flywheel you have? On my 85 Mustang 5.0, I replaced the original 157 tooth flywheel with an earlier style McLeod 164 tooth unit. My McLeod scattershield has both bolt patterns, to work with either size flywheel, but the 164 tooth flywheel has the ring gear located further foreward, which caused the starter drive to be engaged with the flywheel ring gear at all times. I ended up replacing my Mustang starter with a 302 pickup truck starter, which corrected the problem.
Another possibility, is that the older cars had a different starter for standard transmissions than automatics. Back in 1978, I converted my 1970 Mustang (351C 4v) from an automatic, to a toploader 4 speed. My original FMX starter "bottomed out" in the bellhousing, with the starters mounting flange about 1/4-3/8" away from the block plate. I temperaily shimmed the starter out with wahers, but the nose piece soon cracked & broke off. The stick starter had a shorter "snout" where it went into the bellhousing. I`ll bet you have one of these situations.