I seem to recall that on the cores with different size hoses, the 5/8" line was on the "pressure" side, from the cylinder head. They seemed to think a bigger return hose was needed because of pressure drop across the core, sort of like an air conditioner "suction" line.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!