When I bought my 69 Barracuda about 33 years ago, it had a pretty good sized radiator in it. I assumed it was a 22" out of a B or E model. It did JUST fit from side to side, but about 1.5-2" of the core was blocked on each side. I stripped the entire car down to bare metal both times I painted it (once about 30 years ago, and again about 10 years ago). My car runs cool enough, but on a hot day with the AC on, it gets pretty warm. I have a really good radiator with the proper fan housing and blade. Both times when I painted it, I considered increasing the hole size for a larger (22" I think) radiator. I know the radiator support is a structural piece, but it would be easy enough to enlarge the hole and bolster the strength. I decided not to both times. I wonder if anybody else has actually enlarged the hole.
Before anybody asks, the 340 is at about 375 HP, the block was hot tanked, I flushed the CRAP out of the block several years ago with pretty much every orifice open, and after a few minutes I was getting clear water. I have a nice Griffin exact fit radiator that is "Guaranteed" up to 600 HP. Don't get me wrong, it has never overflowed, but on a hot day with the AC going the temp gauge is almost pegged, and the temp at the thermostat housing is about 220. I just live with it.