LA small block timing is suddenly way off

I am betting the timing chain has skipped a cog. The rebuild could have reused the original nylon gears and a new chain. As most of us know the nylon gears tend to loose chunks. I have been there, and managed to get the car running enough by twisting the distributor (ran like a bag of crap) to limp it home.

Please don't "test" exhaust, or anything else for that matter by revving the crap of an engine in park or neutral. It is just plain not good for anything and would absolutely lead to a timing chain skipping.

As was stated previously, with the distributor cap off, turn the crank back and forth and watch the rotor. There should be no, or very little lag in the rotor following your turning the crank.

My two cents worth.