Al, I think what you're looking at is pipe thread, NPT = National Pipe Thread. I checked a few fittings here and I think you need 3/8 to 3/4 NPT adapters. On the manifold side of the block back were the casting flares out for the flywheel half way down there is a 3/8 plug that goes to the water jacket. If you can't find those parts (NPT) locally let me know.
Larry
