Well, It's too late for that truck...

But for the CA guys dealing with a truck with years of back reg and fees; There is a very simple way to get around the fees.
When I was living out there (about a year ago) and bought my 86, it was around 3 years out of reg, and they wanted an insane amount in back fees (over $1000). The nice guy working at the DMV told me to go back to the PO, and have him fill out the Non-Op paperwork and backdate it to the day the reg expired. I went back to the DMV (different clerk) with my title and that piece of paper, and all I had to pay was the back registration cost, NO FEES.
As long as the PO gives you a filled out and signed Non Op slip dated before the reg expired, you don't pay any fees; it was on the PO to deliver that paperwork, and that way the fees don't follow to you.