Don't fold to the insurance Co's.
I work in the insurance business. They work on documentation-documentation-documentation. Show them that this car falls under the "collectors" cars category and obtain documentation showing their true market value, also known as "actual cash value" or "ACV." Obtain this documentation from publications where people are selling the same year vehicle with similar mileage for more $$$ (preferably in your same geographical area). Ask the insurance company to show you the documentation they used to arrive at their assessment of your car's worth. I have seen people use items like the auto trader publications that taylor to their specific vehicle....like the sports car trader, antique car trader, etc. You get the idea. I remember the company I work for paying a guy $7500 for his 1965 Mustang which was a painted (piece of $h!t) bondo baby. The car was pure junk. That guy secured a bunch of different publications with comparable year/models showing much more value that the insurance company originally came up with, and was persistent.
Something else. If you are having the at-fault party's insurance company take care of your damages, they OWE you another Cotton, Precision or V1 or whatever intercooler you had on there. Don't think they owe you factory stock. They have to put you back where you were before the accident, no better, but no worse. That means, not a downgraded cheaper factory intercooler. Show them your receipts, or if you don't have them, ask them to have an independent expert appraiser who knows something about modified cars look at your car and confirm that your intercooler (or whatever their insured damaged on your car) is what you are telling them all along. You may have to pay for a qualified appraiser out of pocket if they don't want to provide one. Lastly, be prepared to sue their insured. When they see suit papers from your "Complaint" filed with the courts, they will take notice. Just be prepared to back your case up to the judge with as much documentation as you can to support your position, and you stand a good chance. They might even cave rather than spend the money and time to fight you in court. It's all a pain in the ass, but the car's worth it. Good luck.