Mike,
I owned (2) '65 fastbacks some years ago, both fun cars and there could be another one in my future as well.
If you plan to build and drive the car, and not as interested in resale down the road, then take a look at Dynacorn.
However, especially since you are in California, I would look for a clean '65 or '66 and start from there. Yes it's more expensive up front, but by the time you are done the cost delta may not be that large. There would then be no issue with having the correct VIN number, and selling an authentic fastback will be a lot easier, and you will probably recoup more of your investment.
Just my .02
