After reading everything, I keep thinking of the impeller slipping on shaft of w/pump when warmed up. Also, fan not mounted backwards? I really thought the new t-stat would’ve fixed it.
Yes, I made my own battery cables to trunk mount battery. Been there since the late 70’s. Never a problem with them. Mine are either 2/0 or 3/0, about 1/2”diameter. Car is at my youngest son’s right now.
Just joined, old member on FBBO. Don’t really know small blocks. Original owner of a ‘68 SuperBee, one oil change done by somebody else in California back in ‘70. Otherwise, all me for everything except body/paint.