SB engine rebuilding got ya! School is Open!

Alway check piston and valve guide clearance with a non stock cam, I had to cut guides and pistons with a tiny cam, apparently the block and heads had been decked a ton in its previous life. I purposely went small so I'd have no problems.

Check the centrifugal advance on your distributor, they can seize in a matter of a day, and I have seen two of the MP distributor's with the adjustable slots, set at opposite ends of their travel, right out of the box.