Worst case scenario, is to have your cleaned and resealed. I think they use steel balls or something to shake out all the crap. But let's see what comes up. Give it a day or more. People will chime in and point you in the right direction.
There are no repro fuel tanks available for 2nd generation Rivs. Your best bet is to have yours repaired, if it is repairable, or replace it with a good 'used' unit. The tanks are the same shape for '66-'70 Rivieras with '69 being the first year for the electric fuel pump which is located in the tank. :TU:
Thanks Angelo, my pride 'n joy - check out the ROA www.rivowners.org when you get a chance. Lots of info there on all Rivieras, will forward some pics ! Good luck with your new/old '66 ! :beer :TU: