The problem is you have a wet joint. If you are trying to go to a dry joint via a jumper hose...what do you plan on using as a tail to block the wet joint? If that wet joint fails...guess where the water goes. Back into your engine. I’d sell the manifolds and buy the right ones.

We used to drill/tap cast aluminum heads and cylinders on 2 stroke engine to add cooling. Never had any issues

I have Gil stainless risers for older wet joint and welded the ports shut. I have one of there billet o2 spacers under them using a closed cooling gasket between the two. I have regular gaskets between o2 spacer and manifold with antifreeze cooling spacer. Raw water side leaves heat exchanger straight to the Gil stainless risers which have been extended to be dry to the tip. I plan on changing gaskets after every season to see how they hold up with primary salt water boating