water circulation question

When mine was together, I simply put a 3/8 1/4 turn ball valve with 3/8 pipe nipples in one hose. 3/8 pipe is the perfect size for 5/8 heater hose.

If this was the 70's-80's when we had "real" parts stores with "real" catalogues, you could look through the "picture book" (pictorial guide" and find all kinds of interesting doo--dads, including cable operated manual valves, etc.