5/8" heater hose and can only find 1/2" fitting

You could go to the plumbing department at Lowes or Home Depot and buy a one or two inch steel pipe nipple threaded on both ends. Get the correct size to thread into the manifold and the threads on the other end will make the 5/8 hose fit much tighter than the stock nipples. You could also coat the inside of the 5/8 hose with #2 (non hardening) Permatex gasket sealer to guarantee against small leaks through the threads.