I have Hagerty for my '71. My experience is:
1) They are wonderful to deal with. Exceptional people and they go far out of their way to make sure that everything they do is to notch.
2) No mileage limits - however, that said, your insured cannot be your "daily driver". You have to have another car/truck that is your daily driver. How many of you don't have another regular vehicle that doesn't need Agreed Value insurance? Likely most of you have, or could have, another "regular" vehicle that would solve that piece of the puzzle. If your truck IS indeed your daily driver, do you really need agreed value insurance on it? Check
www.nada.com/Classic-Cars and see what their value is for your vehicle normally. They are very fair and unless you're driving a really, really nice truck for work, hauling, carpentry, tree trimming, etc., their values may be plenty good. Those combined with comparable sales research should an accident arise and you can make a pretty good case even with regular "Stated Value" insurance - which is the typical phrasing used for the value on most policies.
3) It does have to be garaged at night. I have not found an insurer who'll stray from that, period. They all require them to be garaged (likely due to the ease of theft in this time period vehicle). If you cannot do that, Agreed Value may be out of range.
4) Hagerty, like all insurers, requires you to use a vehicle as it is intended - and as they have insured it for. Hagerty is a "collector car/truck insurer". Which means they expect that you're not taking it to work, hauling chairs, tree and brush, actually anything! They figure a collector vehicle should be "driven and enjoyed" - that's about it with them.
Grundy will allow more latitude in the usage of the vehicle, but any usage that does/can cause damage to the vehicle - like hauling stuff in the bed - will make them raise eyebrows. If you have a truck that you use for hauling/working, make it a truck that can be valued more reasonably under NADA values in their Classic Vehicles section. A typical '72 Chevy 1/2 ton 4x4, in work-truck condition will be more than fairly valued under their value guide. A restored, top shelf '72 will not - that's where the Agreed Value comes in. You can drive it without worry, but it's not the truck you take to Home Depot. Unless you're buying a pvc fitting. LOL.
Another thing to consider is Hagerty is their road side assistance plan that covers all of your vehicles that you own. If ANY of your vehicles breaks down, they’ll send a truck and tow it up to 120 miles. No cost, period. It's an excellent adder to their policy. Cover one truck and all of your cars/trucks are covered and it's the best road side assisstance I've ever seen. Fast, they are very through, and cost has never been an issue. Covered 100%. And 120 miles is a long way for a towing bill. If I had kids still at the house - I would value this highly. If anything happens, you can be sure they have someone to call who'll look out for them. You can't always be there, as much as we would like to be.
I'm super happy with Hagerty, and yes, it's a few dollars more - but my truck is valued very fairly and if anything happens they'll track down period correct parts, work with restoration professionals to repair it correctly, or pay me what I bargained for.
I'm quite sure Grundy is just as good at the policy they offer. It just appears Grundy caters to the owner of many collectible vehicles (over $100k is their niche), and they cater to high-dollar drivers (Bentley, Ferrari, etc.).
Just my experience. With some research, you can nail down a policy that will offer the best protection for your truck, and your usage. Hope this helps a bit.
saa.