I’ve on,y ever used two per port…..

I've had that starter, the 9506 on my car on two different engines for the past 16 years.  It is wired with the jumper wire between the two terminals and then to the solenoid like a stock starter.  It is NOT a PMGR style starter that needs to be wired like a GM.....

I usually look on eBay for the old stock stuff….and I’ve used Mouser. 

Just changing the gears from 3.00 to something like 3.50 would wake it up without doing any engine work........doing those other mods without changing them you might still be not to your liking, it's a heavy car and right now it's got freeway cruising gears in it.....

Wish I could offer more for that but my PPL was back in 87.....given your attention to detail you've got this, best advice I can offer is to relax, go SLOW and methodical, nowhere does anything need to be rushed.  If you think you missed something, as long as the ride is going keep moving forward.  (I fly for a living, so a checkride is a yearly thing, if you feel rushed or jammed up take a deep breath to relax, calm, and help slow the situation down so it's manageable......one task  at a time)

I used studs on mine.  Keep in mind, the holes in the pan aren’t spot faced (at least the ones on mine weren’t) so some don’t exactly sit flat.  I’d recommend using ARP nuts with the flange, there flange is more narrow and fits better in some of the locations……I didn’t have any issue getting a socket on any of them.

Should be a tag on one of the bolts……
https://www.stangerssite.com/steeringboxtagdecoder.html