United States Customs Web Page Regarding Importation of classic/antique vehicle: CBP Customer Service.
I don't know how to import vehicles to the United States and only know a little about importing from the United States to Mexico:
Last I checked the government import fee for antique/classic cars was 90% of the market value. However, there is a local man here in La Paz (originally from Mexico City) who has quite a collection and knows more than I do. He doesn't seem to have a problem finding and importing his vehicles so maybe he knows some tricks. There is also a local man originally from Oregon that has three Ford Model A's that he restored here. I used to see him at the local car shows, but we haven't had a show in awhile. I wonder how he does it? Again, this is from the United States, not to the United States.
For just restoring your USA vehicle in Mexico:
If you are in San Diego you could just drive across the border to Tijuana where there are all kinds of body shops, upholstery shops, etc. and have any work you want done and just drive in and out. That was before the virus restrictions on border crossings, of course. In Tijuana back in the 1970's I had a complete interior done in black diamond-tuck (including dash, package shelf) for $150.00 USD. More recently I had my Dodge Dakota reupholstered and the windshield replaced in Tijuana. Maybe from Texas you could do the same thing in Mexican border towns.
I drove my 3 classic Mopars here in La Paz from the United States with California license plates and I still drive them around here with current California tags without a problem. But Baja California Sur is a little different with respect to tourists and residents from the United States and Canada driving around with "foreign" license plates. The police don't seem to care as long as registration is current and matches the vehicle. I have six vehicles here in La Paz. I imported my Dakota to Mexico from Tijuana for about $900.00 USD in fees about ten years ago. The other five vehicles we drive (3 of which are classic Mopars) all have California registration/plates still.
There are some famous Mexican restorers that I have found on youtube who will do professional custom work probably at half the cost of a US facility. These guys are located in Guadalajara: