Piston slap or scuffed pistons isn't usually that consistent in my experience. Unfortunately I've dealt with it a few times

good news is its only a 350 .. .. just saying ...

Sounds like rod bearing. Diego's has a nice set of 383s for sale. Cut another whole and make twins. Lol

Bent rod, pull it.

I would pull the valve covers and take a look, probably comng out but. I would say lose coupler bolts, bent rod,or valvetrain. To me it does not sound like a rod bearing.

Does anyone know where I can find a short block?

You can buy gm crate motors pretty cheap Change gaskets cam and have a brand new motor for about $1800

http://www.youtube.com/watch?v=K7Z8I0CvEVs

Sound familar? We did the same thing--pulling spark plug wires and couldnt isolate the problem. Checked the valvetrain as best we could and just couldnt locate the problem so the engine was pulled.
It ended up being the number seven wrist pin was bad and causing piston to score.

I've got a drop in ready big block I can sell cheap

\\\\why not just pull the valve cover and check if a rocker arm got loose hammering the lifter. Happened to a friend's 67 Mustang fastback recently. It started with the hammering I could hear driving next to him. Told him to pull over to the curb and pull the cover but he ran it for another 5 minutes and kaboom, rocker stud broke. Luck has it I still had some studs from a 429 at home, so I machined one down to accept his rocker, so we could fix it on the road taking the car home to check the whole engine later.