On my 250, I made a plate of 1/2" aluminum, drilled & tapped mounting holes and 1/4 NPT for the plumbing. I used 3/8 steel tubing tee'd off the heater hose fittings on the water outlet and water pump. Works fine.
One note - my Offy intake had core plugs in the bottom that leaked like a sieve. I JB'd them and have had no problem. Check before you fill it all up with anti-freeze.