I bought a custom swing arm from a different source (before MGP made theirs). Aside from delivery issues, when it arrived it didn't fit either, and shipped without an axle. Got it resolved with some polite, patient, understanding about QC issues when dealing with small shops.
As for MGP, I've bought 6 systems from him. I don't know why, butt each and every time it went flawlessly. Every time he sent my stuff, I sent him and email and commented on it. He responded promptly. First time I bought from him, when my system arrived I called him.
"Hey, its Chris"
"I just got my system, seems to be an issue"
"Yeah, whats that?"
"Its too dang pretty to have only cost $500, wheres the invoice for the balance?"
"Ah, thanks, thats nice of you to say."
"Cant believe they're so cheap and so well made"
"Delighted you like them"
"By the way, theres a set of 105 jets in the box, what do I owe you?"
"Not a thing, thought you'd need them to tune for the system. Just tried to help you out ahead of time"
"Dang, really appreciate that, sure I can't pay you?"
"Wouldn't have it, you enjoy them, and thanks for the call, appreciate it."
Now, I know Chris can run hot/cold as we all do when pressured and stressed, but I have yet to find any small shop that can always be perfect and perfectly well-behaved. Especially when its about a "part" versus a "project" purchase. I get everyones perspective about response time and customer attitude, but that does work in both directions. Call in a tiff, you might get miffed back. Call, be patient, be polite, be accommodating, and you can probably avoid anyone's ego.
Theres no doubt GV's arm had issues. Me, I'd have returned it to Chris and requested it be fixed or refunded. Simple as that. Or, dealt with it myself and been done with it. But I can't expect anyone to make anything for a 40 year old bike frame and achieve "bolt-on" accuracy when the frame is not in front of them. There is another possibility too, GV's frame had been tweaked or modified that altered fitment from original dimensions. Or the arm was simply made poorly. Either way, there's little doubt that its custom stuff and requires solutions in the field. If you aren't up for that, buy a new bike.
Stilltime - sorry for the hijack, buddy. I'd TIG weld a new brake rod tie down to the arm and stick with your drum if you can't find a disc hub.