Wut?
 

Ok so heres a weird done. My engine (95 4.3 vortec) while I was accelerating I heard a boom and something started rattling and felt like the engine through a rod. Turned it off, tryed to start it and it would turn over then lock up. Well, I just now got a chance to look at it (had the new engine put in about a year ago) and I pulled the oil pan off to take a look, everything looks fine. I can turn the crankshaft and it all turns fine. Whats up? No thrown rod or anything broken but it locked up back then????

could have been a stuck valve. was there any metal in the pan?

Take and remove the plugs and see if the engine will turn over.Also check for debris inside the cylinders.Seems to me theres a ring that broke on the piston.Not to give you bad news but you may end up needing major engine repair or replacement.I agree with twip a suspected dropped valve could be the problem also.

I think he replaced it already, so a R&R was already done.

No metal in the pan, everything's clean. (its on an engine stand) I had it replaced a year ago I'm just now starting to rebuild the old one. I'll take the heads off and see if a valve broke.

So I got the heads off today and nothing was in the cylinders and the heads were fine... What else could that have been?

Hemm very strange seems to me you have look in all the places where the noise could have came from.Did the engine lose oil pressure when this happened?The only thing I would now suspect is that the distributor shaft broke the connector that connects to the oil pump shaft.

Did not loose oil pressure and the distributor shaft did not break... baffles me too

This engine has a balance shaft my last and only suggestion would be to have a look at the bearings that hold in a place.
Also have you taken the timing chain cover off and had a look inside there?Maybe the chain has slipped or the gears have broken a tooth /teeth off.
Ohter than Im out of ideas except to replace the engine.Good luck let us know what you find.

Timing chain and gears are fine. Still don't know. Going to call my mechanic who swapped out motors and ask what he thinks.