If you want the best possible mount, take the time and effort to bed your mounts.
This is done by finding a chunk of tubing the same size as your frame and wrapping it with sandpaper. Then use this to sand your mounts to the exact angle needed to mount it solid. This is BY FAR the most secure way to mount a bike engine and with better studs will give you years of trouble free service.
My Pig has had at least three different motors and the paint under my engine mounts is still intact. This is because my angles are dead on.
Ditto: You can also use a 'sanding drum' chucked into an electric drill to grind in the proper angle. I'm not a fan but others have used epoxy based fillers to bridge any air gaps where the engine mount saddle meets the frame tube. I prefer metal to metal and shy away from epoxy. Your choice but if you opt for the epoxy method here's a hint:
Wrap the frame tube with waxed paper where it will contact the engine. Spread a liberal amount of epoxy in the engine mount then bolt the engine into the frame. The eopxy will (hopefully) fill any gaps and won't stick to the frame because of the waxed paper. After the glue sets you'll have a gap free installation. Just make sure the engine is clean and free of any oil or grease where you want the epoxy to adhere.
I'd rather see you sand/grind the engine to match the frame angle. That's the best way. Whatever, don't drill a hole in the frame to mount the engine.
Tom