How do you know if you have "enough" radiator?

Is there any rule of thumb on the radiator size to horsepower ratio?
I'm not sure if my radiator will be enough for my impeding motor swap and I'd appreciate any feedback.

Back in the 1990s I bought an "upgrade" radiator for my 318 Dart with the anticipation of swapping in a 340 or a 360 when I finally had the money. This was supposed to be a stock replacement for a 340 car so I assumed it would be enough. I didn't have a lot of money back then (nor do I now) so it's a basic radiator (not aluminum) with metal end tanks at least. It's significantly bigger than the laughably small stock 318 (non A/C) one...but it still looks like a non-high performance car unit.

Assuming it is indeed a stock equivelant to a 340 car, should it be adequate for a tuned motor?

I drained and pulled it last night, bringing it in to be boiled/tested...I plan to run electric fans. I know a good radiator can be relatively affordable but I'm on a shoestring budget...so every penny counts.

Thanks for any thoughts!