slant 6 troubles

First off, it's not the block. Nor is it likely to be an exhaust lobe. Intake valve perhaps, exhaust no.

What is you vacuum reading at idle, and is it steady? Is it crosswired? 1-5-3-6-2-4 Plug wires and distributor cap new? It would be unfortunate if you had spent all of this money only to discover that a $5 distributor cap was your problem.

I'm sorry, but if your mechanic has done what sounds like a complete valve job on a slant 6 and it still isn't running right, HE IS A DUMBA$$. Take it somewhere else, and sue the bastard out of business.