I had a feeling you'd run into these issues with the fiberglass decklid. They are fine for a race car, where weight is the primary concern and fit is secondary. It is always a ton of work to get them to fit like an OEM steel decklid. Just return it, if you can.
Cutting a notch in the original decklid isn't "the end of the world". Just make sure you save the piece you cut out. There are lots of good sheet metal fabricators out there, that could weld the notch back into the decklid, so no one could ever tell it had been cut out.
I'm surprised that you needed a tach adapter for the HP. My tach worked fine without an adapter, with my Holley Dominator ECU. I assume the tach output on the HP is the same. Oh well, as long as it works!
Hopefully, we won't have to wait too long for a posting that has a video attached to it, with sound!