As far as I know, the significant difference between the Boss 302 and the Cleveland is the water passage in the head.
The Cleveland has it blocked in the casting. I think all you need to do is fabricate a plug to sit in each head to block the 302 water passage.
You probably will need a machine shop to do that for you since the plug is what will need to be machined and the water passage will need to bemachined round and countersunk to accept the plug.
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