Well, I agree with your mechanic. The Gray Marine engines were nothing more than Detroits that were "marinized" for the U.S. Navy . The same thing Johnson and Towers did for pleasure craft.
There are still plenty of them floating in marinas and quays all over the world. The problem is finding one. There are probably more than a few in derelict boats right in your area.
There used to be a guy here in San Diego that had a yard with literally MOUNTAINS of them piled up. But it's been so long I can't remember his name. I will ask around and see if he's still in business but if you haunt some marinas and ask around you might just find an old boat with a rebuildable engine in your own back yard.
YOu could take out the Detroit and scrap the hull.
Old trucks and especially old buses are another good source for finding a useable Detroit long block. There are surely thousands of them just sitting in fields and behind barns that wouid probably fire right with fresh fuel and a hot 8D. The problem would be that your manifolds and such might not transfer so well between your VERY old Gray (1940's?) And perhaps a newer casting of the Detroit. I don't know for sure. You will have to find new bolt ons if that's the case. Just something to keep in mind before you go down that road.
So, don't give up, keep hunting!
Good luck.