318 timing
 

I have a 74 318 in truck I am building
I believe timing spec is TDC . With timing set there idles poor backfire through exhaust. Set with vacuum guage method runs sounds great. When checked with light it is advanced 35 btdc. Has edelbrock 750 and performer intake, pro comp ignition and headers. Can it be ran that far advanced or set back closer to spec truck not together enough to test for pinging

Welcome. Is that timed with vacuum advance disconnected and plugged?

I would:

#1-Turn harmonic back and forth until resistance felt (that's trying to move valve springs), easily done with fan blade or v-belt, wrench on balancer is best. 5 degrees ok, 15 or more should be replaced.

#2-Use a spark plug piston stop tool to check TDC mark on balancer.

My bet is on a slipped (moved) timing mark. Do the TDC locate with piston stop for sure.