some of the Idaho guys just keep the vin (any vin) on the tube somewhere and register it then insure it that way.
That sounds good in theory, but what happens when a claim is filed and the insurance adjuster wants to see your "1990 Jeep YJ w/ VIN#blah, blah, blah", but its simply a YJ frame with a mess of tubes welded to it?
Or in the case of theft, are they simply going to give you the current market value of that same "1990 YJ" (the one you've got 30k into it), then you've got to fight with them over aftermarket parts and modifications? (still better than getting nothing, I suppose...)
I'm not saying that there isn't an ideal insurance policy out there somewhere, but there are going to be potential "gray areas".
No offense to any insurance guys out there, but do you think an insurance company cares about being fair to you? They are in business to pay only on claims they "have to" and at the lowest cost. This is where they may point out those "gray areas" regarding your rig.
The most important question is this: Has anyone here filed a claim in such a situation and what was the outcome?