Yeah - Been through this before:
If you did not validate modifications with insurance and specifically write them in as riders to ensure coverage for the value - All you can do is make a case for a higher value based on similar cars for sale.
They may also cover any repairs recently made (6 months?) if you can show receipts.
Unfortunately it's most likely going to be a losing proposition - but certainly worth the effort to get any of the added value back.
I hate the insurance part - as there's also the emotional tie in these cases which is worth nothing but heartache....
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1996 Z-28: Bolt-Ons, Harwood Cowl Hood, 18" Black Z06s
2005 Subaru Legacy GT - Stage 2 - My DD
2012 Audi S4 - The Wifee's Ride (Wait - the wife drives the fastest of the stable?
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