Do a leak down comp test with oiled cylinders to see whether a valve is letting by.
If it is then I would say number 5 exhaust valve. Is that definitive enough for you
My 502 suffers from reversion which starts out as slight pitting to valve face followed by complete meltdown of valve when it gets hot. If you're running the same sort of cam overlap you could have a similar problem.
As JF pointed out elsewhere blipping throttle before turn off can cause the sort of reversion which sucks a tad of saltwater back to sit on the valve face. Minor rust spot forms and then fuel ignition causes a massive increase in temperature locally on the valve face which in turn leads to catastrophic failure of the valve later.
Other than that you need to know what 454 it is to understand whether you have roller lifters etc