I would say they are tuned to the vehicle in as much as they are crash tested that way. Big issue with the 62 rear belts is that they use a different mounting system, two bolts rather than 1. I think you could probably take the assembly apart by drilling out rivits and make it work. The liability is in the making it work part. That's the part the insurance company wants nothing to do with. Toyota has test data backing up their mounting method. Joe Blow autobody does the bracket for the insurance company, what data do they have saying they conformed to test requirements. Being an Engineer and after enough training from the companies product safety group - you get used to thinking of everything in terms of 'How will this look in court with a burned, paralyzed or dead child?' I know it sounds mellow dramatic, but professionally I always liked having UL or CSA standards telling us how hot something could be, because as long as we met them we were 'safe'. Of course what you do as an individual is different, but I totally see why insurance companies and bodybshops would just say no. Heck look at all the roll cages that are sold as 'show bars' because of th implied liability.
I've also said all along that I've over sensitive because of two major crashes in 14 months. Or perhaps because of two crashes and too many close calls where my thinking ahead about what stupid thing the guy in front, beside, or behind me might do was all prevented another accident I am appropriately sensitive. I've stopped riding motorcycles on the street all together. The rear ending I would have been taken away in a box after my parts were scouped up. This last one, I might have just crawled away crippled.
I don't go around telling others not to ride or not to drive a 30 year old vehicle with modified seatbelt parts. That's never what this thread was supposed to be about. I never actually got an answer to the origonal question - at least not in advance of insurance actually totaling truck. Typical Internet I guess. Opinions given on everything but the question asked, all kinds of advice on what I should do, but no facts on what insurance required to do. Not that I don't appreciate everyone's well intentioned comments, they just didn't help me with my insurance question.
Interesting side note, because I knew of the limited availability of driver's seat belts, I went ahead and ordered one when insurance company was saying no way they would total it. So I now have one of six or seven nos seatbelts in the country, and no truck to put it in.
Insurance company got real pissy with me about surendering the truck right away because they aparently forgot that it was at the body shop racking up storage fees. I explained to them in an e-mail they should be thanking me for not authorizing the body shop to start work or they'd have a $6,800 bill they'd have to pay the shop on a vehicle they would still have to sell salvage. And of course I only took it there in the first place because they insisted I do so. Idiots would have been money ahead had they just totaled it in the first place. I gave them fair warning on the belt issue. Instead they were bullheaded and they paid a thousand dollar storage bill plus I'm still in a full size SUV rental going on 37 days. I think even at the insurance disount rate it's around $50 a day. Doh!