On my Schwinn I drilled a hole through the frame and put a spacer and a flat plate on the spacer. The flat plate bolts to the front engine mount. I don't think I have a picture of it on this computer. Lots of different ways. Cruiser might have some pics of the way he did his on the forum. He'll chime in after while...................
I would still go with the muffler clamp if I was you. I don't have a picture either but there is one around somewhere. The clamp is a u bolt on one side that would wrap around the frame, then a channel bar on the other side that is u shaped on one side to grip the frame then flat to bolt up to the engine mount. You would have to drill some holes and buy some replacement bolts for the studs in the engine, but it would save your frame. It would also be infinitely more adjustable than a hole in the frame as well.
I have never used one but if I had an over sized frame it's what I would do.
I think the way to do it is to loosely hook the seat tube end, then move the front of the engine around till you get the best seat for it. That will require that you move the rear (seat tube) up and down as well. I use a little auto heater hose to fill the gaps. I have moved my motor from one bike to another so they had to be reset both times.
Here's how I did mine. It's not that great of a picture but it's all I have. If this won't work let me know and I'll go out and take a pic of just the front mount. But that may take me a day or two. Hope this helps.............
another option to the muffler clamp would be a tower standoff. I used them on both my builds. They mount to the engine and attaches via stainless steel hoseclamp. Haven't had any issues with them so far. I used heat shrink to the bottom, so not to scratch the frame. The standoff is .25", so no worries of flex or warpage.
a couple of hints when using a standoff... you will have to grind a little off the inside edges to clear the bolts from the engine. Trim the engine bolts about .75" . For the stainless steel clamp, use .5 x 4" heat shrink, it will cover the clamp and protect the frame a little. If you crank too tight on the clamp it will strip.