I got a quote from the GMAC website then called a GMAC representative to actually sign up. I was honest when I said the bus had been professionally converted since I get paid to fix/move/build stuff (I only have the seats out so far,but a professional did it, lol). His quote was $35 higher than the website quote so I politely told him I had another company to check then went back to the website to actually signed up there. Final cost was $216 for 6 months of liability only for my E-450 shuttle bus.
The website didn't ask who did the conversion but did mention that there was a discount for the time the bus was "securely" parked in a storage area. You have to call in to a service rep for details on that.
One really goofy thing was that if I went to www.gmacinsurance.com
I could sign up for insurance but another link told me I had to call a service rep. Very weird.