I appreciate the info on here from the other posters, I'm going to have this problem in the future too.
I have about a dozen vintage cars with Hagerty, and they've done great, but I asked them in advance about my current project, and they told me they won't be able to cover it.
Kitman coupe I'm building as a daily driver/thrasher, (bombproof drivetrain/chassis built the way I always wanted a simple, no-frills personal car to be, will see salt, outdoor parking, offroad, hard use, etc. so I'll do galvanize dip, nickel flame-spray and liberal stainless) and Hagerty noted they can't do it since its homebuilt by me, not VM, VS, IM SE 'production' shops.
I still keep one winter beater insured with State Farm, as Hagerty requires you have at least one 'normal' car, even if you use your vintage fleet a lot. Costs $120 a year with State Farm, but it fulfills the requirement.
Years ago, I used to have all my vintage stuff with State Farm, but when the value of VW Microbuses started skyrocketing, I updated the 'stated value' from the $5K they had been languishing at for years, to the current reality $40/80/100K of 21-window deluxes. Their first salvo was to charge me $400/month for the 21 Window I drive Chicago - LA on Route 66 every summer, but denied my stated value of $45K. I asked about than and they replied with their "Kelly Blue Book Value" of a '67 21 Window Microbus as only worth $500, and told me to get an 'independent appraisal' if I didn't agree with $400 a month on a vehicle they pretended was only worth $500. So, 10 cars, $250 each for the appraisals, and they all came back HIGHER than my valuations which State Farm initially disagreed with. So, after spending $2500 on appraisals that only reaffirmed what I already knew regarding what my cars were worth, they summarily dropped my vintage fleet.
The thrasher Kitman coupe will probably fall into every insurer's uncomfortable zone: homebuilt, daily-driven, parked outdoors, unlimited miles, $50K+ value.