I did something similar when I was building my car - My work job was ramping up in hours again and I had gotten to (a.) the point where I didn’t have much time to work on it and (b) where I needed a body shop to do a few things I wasn’t good at, like a killer paint job. I dropped it off thinking it might take 3 - 4 months and kind-of forget about it. I would call once in a while to see how things were going (or not going, to be honest) and got some famous excuses, but there it sat for a long year and a half til I finally called and said I would be by in two days to pick it up, as-is, to finish it elsewhere. I was probably my usual gruff self on the phone because the guy calls back an hour later and says that if I could give him four or five more working days on it, he would get it done for me, and he did. Good thing, too, because I was unable to find any other place that would touch it, even a couple of local Hot Rod builders.
So my only advice is to have a heart-to-heart with Greg. Tell him you would like to have him complete your coupe because you trust him to do a good job and that you’re cheap advertising for him. That you understand that it may take some custom work to do that, here and there, but that you’ll work with him to make it as un-intrusive to his shop flow as you both can so that he can treat it as just another build. Bear in mind that you’ll be starting at the front of the (long) queue line but at least you’ll be in the queue.
And finally, just ask “What’ll it take to get in line for shop time to get this done?” And go from there. A lot of us would like to see this come out of Greg’s shop, too.
Good luck!