[AZKen]

I realize you live overseas and shipping is way high along with whatever tariffs are doing. So maybe you sourced that Chevy starter there. Not sure what country you are in now.

I believe this endeavor has too many issues to work. I know you are handy and quite knowledgeable in mechanical design. Me too. Whatever the correct GMC starter was used was good then and if refreshed will be good now.

Your issues, if I understand correctly, are quite likely:
1. Battery power, I.E. breaking down under load, need more amps, More CCA's, etc. Get a big CCA battery and make it fit the holder.
2. Corroded, dirty or loose cables.
3. Ground cable breakdown. Tripple check. Things get old or messed up in certain climates and don't get noticed.
4. Battery cable gauge too small. Get larger ga.

Don't know history of the issue, just started happening?, was good but gradual slower crank? truck won't start sometimes? crank speed faster at times and slower at times?......something new done to something?

A "bit" slow is good news. It means my list is the answer. All these "little" things will fix slow crank if starter motor is refreshed.

Stop before you screw up your only in-country GMC starter.

FYI: The power of a starter is in the number of field coils. 2, 3 or 4.

Anyway, this is my RSVP,
Willing to help.