I'll support that GM crate suggestion. I've got Juan's old motor in my K5 now, and I went through it before installation to add a hotter cam and my SR Torquer heads (reworked by Gruelick, VERY good!). I really just needed the short block after my son put a rod through my old block, so I bought that engine from Juan to help him out. Anyway, the stock long block ran really well in Juan's truck as it was. Much better than your typical 350 for the era. When I got into it, I found several things I wasn't expecting including 4 bolt main (ok, that was no surprise being an MD), powdered rods, forged crank, and indications of internal balancing. I'm running a roller cam, and this block came with provisions for the bolt in lifter spider (found used) and front retaining plate. So I went to GM for the front plate and got a standard plate, but it doesn't fit. Went back with a measurement and found out I needed one for a ZZ3/ZZ4 HiPo engine.(?)
What does that all mean? No idea. Their books show that cam plate specific to only the ZZ3/4 but maybe it’s just the block they had lying around that day? <shrug> I’ve been meaning to check the heads that came on it to see what they are, just never think of it when I have the chance. But anyway, if that's what they sell for a $1300 factory crate long block then it's a screaming deal. I will say one thing, that thing as it sits in my K5 is a beast and runs smooth as silk...