Have you provided them evidence of the upgrades? Have they provided you a copy of the CCC or ADP market evaluation they used to determine value? if not, request they give you a copy. Review it carefully to verify the vehicle condition and equipment were properly included and rated.
Normal maintenance and or repairs do not increase the value of the vehicle, upgrades will. Upgrades include paint, custom wheels stereo equipment and such.
If all else fails, demand arbitration of the dispute as outlined in your policy. You will need some evidence to support your case. Evidence includes prices of similar vehicles sold in the local area and or an "expert" in used vehicle pricing. They will provide their own "expert"opinions to the arbitrator who will decide how much they owe you. An arbitrator can be any mutually agreed individual, a judge, an attorney, an appraiser or a used car salesman.
Keep in mind, many policies exclude custom equipment like the wheels, hood, bed cover and stereo.
You may want to look into settling the case with you retaining salvage and then repairing the vehicle yourself. If you choose to do retain the salvage, the insurance company will deduct the salvage value which is typically 15-20% of the offer.
Good luck.