You do not need to seal the IC to the Rad! lol
I have a huge gap between mine (maybe 3") and have been out today which is 28deg blasting around at 25psi (circa 270bhp) and 6000rpm+ for over half an hour and am not anywhere near overheating.
If your timing/fuelling are correct, your cooling system is working fine (not old and clogged) and your head is seated properly on the block you should have no issues at all!