obviously the car wasnt originally a 455...so was it a 400 or 350?
if it was a 400 you should have the fillers...3 sets...might be missing ,and if it was a 350 would be missing if not added by prev owner..

1) X fillers, metal plates at top in front of radiator
2) masticated fillers or rubber, on ea side of bottom of radiator
3) lower air baffle


then of cousre , check timing, and if carb runs lean it will heat up...check carb...I think besides the fillers most has been covered...
my problem was a missing fillers...also a prev owner had added a helper fan , a pusher, that slowed air flow more than helping...