starter motor for slant six

Something Dan did leave off is that from the early 60's through the Denso units, starter speed gradually went up

Naw, I didn'tÂ…!

quite a difference, though the '87 is not a denso starter.

The '87 has the 1.8hp long-frame starter with 2:1 gear reduction. The '73's original starter would've been a 1.3hp series/shunt short-frame unit p/n 3656 650 if it was early production, or a 1.5hp 4-series short-frame unit p/n 3656 575 if it was late production. Keep in mind also that all the short-frame field coils can be installed in all the short-frame field housings, so if the starter's ever been across a rebuilder's bench, it may not have the same field coil configuration and size as it originally did. Your '73 sounds like it has a 4-series unit; cranking speed is faster than with the 3-series/1-shunt. For comparison, here's a 3-series/1-shunt short-frame starter cranking a V8 (a 2095 150 in a '66 Monaco).