To drive it without restrictions would require carrying regular full coverage through a normal car insurance company.
The problem with that is, they'll use the N.A.D.A. guide or Kelly Blue Book to value your car, both books are typically way off when it comes to older Mustangs for several reasons.
Here's a few good reasons not to stick with regular full coverage:
1. The books don't take into account region or market value (my 1987 Mustang convertible had full coverage on it, somebody backed into it and did $3000 worth of damage, book value was $1500... car totalled (I kept it anyway) what REALLY sucked was that I had a buyer that had offered me $5000 for the car two days before the wreck and showed up the day after and didn't want it anymore.)
2. The insurance company won't take any modifications into account unless you've got receipts (this sucked for me because I'd built up my '87 on a budget, trading work for parts, buying used parts where it didn't matter (headers, H-pipe, etc.) and bought pieces on Ebay at substantial discounts compared to the actual value of the parts.
3. They can, and will, use any modifications against you if they can. In another situation where a St. Bernard had jumped out of the pickup in front of me on the Interstate, the insurance company refused to replace my wrecked H-pipe because it wasn't a street-legal piece. In fact, they refused to fix or replace the street-legal exhaust components that were damaged too just because of that H-pipe.
If you go with a classic car insurer like Grundy or Hagerty, you'll be restricted from certain usages of your vehicle, but if some jackass backs into it in a parking lot, or some idiot runs a redlight, or some dummy on a cell phone rear-ends you, you'll actually get what the car is worth back.
Trust me, as soon as my '76 Ghia is restored to a certain point, she's going on Hagerty. I've got liability only right now because paying the extra $1000/month for full coverage simply would not do me one bit of good from my prior experience.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
Which insurance company offers Full Coverage for my 1966 Mustang? 
