LA small block timing is suddenly way off

It sounds to me that there is no proof that you have a timing issue.

You said that the engine is pretty fresh which makes me think that a jumped timing chain is unlikely.

You have checked for proper firing order, changed plugs, etc. Great. - Often it's the simple things that bite you. - Good to rule them out.

You also stated that the problem may have started after revving the engine to check out new exhaust system. I think I'd pull your valve covers and make sure that the valvetrain is in good order. I've seen guys send a pushrod through a rocker arm when taking an engine to high RPM and the motor hiccups on them. Even if it doesn't damage a rocker I've heard of it bending a pushrod. I'm not saying that it's likely but it has been known to happen.

At any point did you disconnect the wires to your distributor? Even though the MSD will run with the wires reversed, - it will run like crap.