I was with Hagerty for years, and had their coverage on my modified 68 fastback from the time we had the body ready for primer. But, Hagerty got ridiculous with their costs for insuring a modified car that wasn't even driveable yet, so I checked with Heacock and Grundy. Heacock would give me no more than 100 miles a year on my 68 coupe for driving, which was ridiculous. The cost wasn't all that great, either. I may drive my coupe less than 100 miles a year, but I don't want to be restricted like that.
I went to Grundy and they gave me great coverage at a good price and no hassles about my modified car. They even agreed to bundle in my enclosed trailer and they don't normally insure trailers. To insure my modified fastback with Hagerty, I had to fill out a very detailed form about each modification. When I got the quote from Grundy, I asked do you want the information on my modified car and they said no, just tell us what the total car is worth.