I just ended up drilling new holes in the base plate to match the original holes in the hull. All the screws are firmly seated in the metal plate under the trunk. Took it out this weekend and it held strong in 12 mph winds. I'm glad I didn't end up having to fabricate an adapter. It's so much smoother than the old block, and I replaced the one attached to to boom as well. I had to splice a new main sheet into the boom block though, the new hardware only takes up to 1/8th inch line. I had a much thicker sheet in the old hardware. Not a big deal but it does feel different with a thinner sheet. All in all a great upgrade and thanks to everyone that gave me advice!
Edit - Not 1/8 line, more like 1/2 inch. Sorry my mind was elsewhere. But my last mainsheet was closer to 3/8, so the new one is quite a bit smaller.