If the pistons are too far down the hole, the heat can't escape as quickly. What heads are you using? If the heads are the smaller chamber size, it may be creating too much compression for those pistons also.
As Vikki mentioned before, a 455 is harder to cool than a 350. More heat is being generated but the same cooling system in the block and radiator is being used. It's even more important to have things running as efficiently as possible. The less efficient your system is, the more heat it will generate. If the timing is off a little, it will make a difference. The "ping" theory probably brought you closer but I doubt it is at it's most efficient point. You could probably get away with the "ping" theory on a 350 and maybe 400. I would get a timing light to be as accurate as possible. I think the distributor you are using and the lack of vacuum advance is part of the problem too. That vacuum advance will help with cooling when every little bit of cooling is needed on the 455.
It is important to have your harmonic balancer working right at least to prevent damage to your engine. Harmonic Balancer
Check the top post on cast impellers for a good cooling tip and Running Hot on the HiWay about timing, etc. You could also search some of the other posts.
PY is a great resource for info. FGF is just a percentage of Pontiac owners and PY has many Pontiac owners with valuable knowledge. I figure it's better to give you a link than to repost everything ...lol. I hope this helps.
"An ignorant man thinks he knows everything, a wise man knows he doesn't."
