Gary Hall once told me the Phoenix model was made with TWO std. thickness copper cores on regular tanks. He once donated one to a 'Vegas Fun rally raffle as the grand prize; I picked it up and held it up to the light. It was indeed heavy (over 50 lbs) and no light could be seen through the core!
Doing sucker fans with a straight-up mounted rad of any size or type, the main drawback is you will lose some skin off your knuckles in getting the hose clamps tight and fan wires connected. But it can be done, with some pain. Better to lay the rad fwd, for convenience & less blood loss. Don't forget when laying a rad fwd, you need a half-size top rad cover to block off air leakage.