[VetteVet]

I'll add my .02 to that GM is right most engine bolts are coarse thread and either 7/16 , 3/8ths or 5/16ths. Power steering pumps mount to the front of the engine either to the block or the heads and use 3/8ths x 16 coarse thread bolts. Some brackets bolt to the block and use bigger sizes and the pump then bolts to the bracket.
A way I use to find the thread size is to try a tap in the hole and when I find the one that fits I read the size printed on the tap. As far as length goes then just measure the thickness of the bracket and add enough threads to get three showing through the bracket. That's an old air force standard.