Sorry about your troubles... I often wonder what would happen in the same situation on my truck.....
I am a insurance claims professional licensed in the state of Tx and Claims Manager for over 16 years. Your situation is a pretty common one when somthing different than a stock late model car is involved. Treetop and White Elephant have some good information for you.
I agree, behave yourself and be as patient and professional as possible. What you are dealing with is an adjuster, who is uninformed about classic chevy trucks, trying to get your square truck to fit into their round hole evaluation system.
It helps if YOU know what your truck is worth and have the figure in mind that will satisfy you. If you decide 5k will do it, then 15 to 20% of that would be the salvage value. Let them know, "I'll take 4000.00 and retain the salvage". This gives them something to work with and they will at least know that you are reasonable and you should have info to back it up. You should insist on them considering additional comparable trucks, provide all of the reciepts and upgrades to your truck and then be the nicest squeaky wheel you can be.
Your goal is to appeal to the highest level of management that you can.
They are often more likely to make exceptions and/or have the experience of similar claim situations to judge a fair resolution. Get your info together and continue to elevate until you get to, at least, a regional manager level. You simply have to ask them to elevate to their manager, or you can ask:
who is your supervisor, may I have their phone number, who is their manager-may I have their phone number, who is their manager-may I have their phone number.... get the idea?
If all else fails:
In Texas you have the option of petitioning the Board of Insurance. This "complaint" makes it's way, typically to the highest levels of your area or state. And they are required to answer it.
http://www.tdi.state.tx.us/consumer/complfrm.html
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Are you feeling sore at all from the accident? Looking at the wreck, you at least had some soreness, stiffness and minor discomfort. Though you might not consider that an "injury", you may have a legitimate claim for your bodily injury, inconvienience and pain and suffering. You should of, at least, been asked if you were injured.. Sometimes a company will leverage the bodily injury claim with the physical damage settlement.
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feel free to PM me- I'll give you my phone # and we can talk.