I forget how the welder cable size goes. Is #1 cable larger than 0 or 00?
I would look at voltage drop at the battery cable at the starter and see what you are getting there.
On most batterys I test with a little portable load tester. The problem with the small tester is on large batteries, the load tester is not large enough to show up a bad battery.
A cranking voltage drop test should tell you if your battery is having trouble keeping up, or if the cable is too small.
I'd test at the starter, then at the battery. If way low at starter, but not bad at battery, the cable is probably too small for that load.
I don't recall the specs for drop, perhaps someone will chime in with that, or you can find it in a book.
When hot, I'd also crank with distributor disconnected to see if the engine is kicking back on the starter.
Check my web page for First Gen Camaro suspension info:
David's Motorsports page
First Gen Suspension Page
67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer