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

The answer IS:

The way you know if a radiator is enough for a engine.....is measure the coolant that comes out of a FULL cooling system (block/heater core)....then you need 2x the capacity of the block/heater core in the actual radiator CORES/TANKS.

Ideally you need an assembled system to measure this out like I described.

But the general rule is.....a 2 core radiator will cool a SB upto 400hp....and a 3 core will cool a SB or BB over 450hp.

I dont recommend running a Side to Side Flow 1 core radiator.
Instead, I recommend a top-bottom, 2 or 3 core for high performance applications. Champion Radiators have a good review, and dont cost a arm and leg......I have a 3 core and if I remember correctly I paid $235 shipped for it with (2) electric fans included....it fit like a glove...and works ok....no complaints here.

Heat soak and thermal radiance, will heat soak a Single core too much to really pull the heat from it effectively for a street car...unless its pretty big. (surface area)

Bottom Line IS: if your 340 replacement radiator is okay....you should be fine.