so it is 4 gauge
and in my truck the battery is under the passenger floor pan, so it's a pretty short wire run to the starter also
my ground is solid right to the frame 3 inches away, and I have numerous additional ground straps linking the engine to the frame as well.

I really feel like it's a starter quality issue by the way it reacts.
so my next question is, which might be dumb, how do I tell for sure on a site like summit which starter will work?
when I go on there it has options for numerous generations of big block and I'm just not sure which mine is.
It was out of a 71 chevelle and it is a 402.
all the searching I do yields all kinds of info about different vehicles and years and I just can't zero in with confidence. Unfortunately I don't think I'll be able to see a casting number in the truck as tight as it is, but I could probably search through my notebooks from when I built it and find it written down somewhere if that's the only way. I'd prefer not to do that as I built it initially 15 years ago and I've moved twice since then so it will be a long search for my old books.

I'm just worried because in the past I've put starters in it that would not engage the flywheel at all, the nose would just pop out and spin like crazy in air, and not even grind slightly. . . ..zero contact.