My 502 MPI with 400 hours backfires intermittently . I cleaned the body and replaced the IAC valve. Any ideas on how to cure this problem?

First you need to do a leak down and/or compression test to make sure the engine is mechanically sound. I suspect you may have burned a valve. If everything checks out mechanically, then I would look at the TPS. Good luck, and keep us posted.
Eddie

+1
I just went down the same path, had a intermittment backfire and started blaming Iac and tps for the prob. Did a comp test found one cyl down 20 psi and the leakdown test condemed intake valve. pulled head and had the intake seat cut and was back on the water trouble free.