Dumb question would comprehensive insurance cover it if it is for sure the head gasket? It is supposed to cover water damage would that include from a head gasket that is a poor design by the manufacturer if it causes engine damage?
I'm sorry to say, your comp insurance is most likely not going to cover this. I would look over my insurance policy, before I would call and ask, so's to not give them an excuse to raise my rates, even if they did cover it, you would still need to meet your deductible. Comp is usually for stuff like a broken windshield, or hail damage, something you had no control over. I would suspect that the "water damage" they cover would be something like, a limb falls off a tree during a storm, and punctures your roof, and rain water damages your carpet, not water damage to the engine from a blown head gasket. The only way I can see that your insurance would cover this, is if you could prove that the board you hit damaged the cooling system, and caused the overheating that caused the blown head gasket. Honestly, I don't see that happening unless the board damaged the radiator, which you don't indicate that it did. A compression test will tell you if you have a blown gasket, or a cracked head, but until it's pulled apart, you can't know for sure which it is.
In my opinion (which is worth exactly what you paid for it), you have three choices, 1, find out what's wrong with your engine, and fix it, 2, get a used engine put in, do it yourself, or pay somebody, maybe get in touch with a local Mustang club, if there is one, and see if any of their members would be willing to help you, I'm sure many on this site would, if they were close enough, or 3, sell it, and put the money towards another vehicle. You have a vert, and, if in as good a shape you say it is, even with a bad motor, you should be able to get a reasonable price for it.
(I know I'm going to catch some flak over this, but I'm being practical here) A 25 year old car, no matter how low mileage, is not a very good choice for primary transportation, many things deteriorate due to age, not miles, (yeah, I drive my 95 GT, 180K+, everywhere, but I have alternate transportation to drive on a daily basis), used Toyota Corollas are a dime a dozen, and practically indestructible. I'm not shilling for Toyota, and have never owned one, but I have a friend who is an accountant, inherited the family farm, disappears for weeks at a time camping in Montana or Alaska or God knows where, and drives nothing but used Corollas, even though he could afford anything he wanted.