I have my 1979 XD insured with Shannons for $17,000
Started off at $14,000, but every few months I'd slip a few extra receipts in and slowly get them to jack up the insured value.
They wanted me to take it in for an inspection (quick car park lookover) and it was all fine. I just stapled $25,000 worth of photocopied receipts on the application. I have a 65% no claim bonus and pay a $612 premium ($51/month) with salvage rights. I guess if I was 21 with a ZERO% NCB, then I'd be up for about $1360/year.
You'd be lucky to find another company so understanding about car mods. I used to be with Vigil, buy Shannons were cheaper. I don't recall your car having to be a factory GT etc to get insured with Shannons.