help figuring out problem

I built a little 327 for my 65 vette. Forged SJ crank, scat I beam rods, 11-1 TRW pistons, probe 1.6 shaft rockers, lunati solid flat tappet cam. 292 heads with 2.05/ 1.6 valves. I degreed everything, checked P/V clearance etc.... Ran the engine about an hour on my test stand. everything fine. I put a couple hundred easy miles on it yesterday. today I dropped the hammer a couple times, shifting at 6800 RPM etc. i pulled of the highway and the idle wasn't right. drove a mile and realized the engine was getting hot. popped the hood and i had thrown the belt. easy fix but now The passenger side pipe has a flat pop, like I have a dead cylinder or something. pulled the plugs and the all look the same. whats my next diagnostic step.