Well, you didn't say "65 what" if you have a Rivvy, it will be much simplier that
the swap I did on the Wildcat, etc etc. I used Rivvy rotors and calipers on the wildcat spindles, making a 5/16ths spacer to hold in the rear bearings. I ordered the caliper
brackets from "Scarebird". with one bolt head issue, they worked fine.
you can call them and order specifically for your Buick and get a list of donor cars.
Do a search - in the upper right corner "disc brake conversion" and you will get an extensive thread on the swaps that Buick Team has done.
64 Wildcat 445
64 LeSabre 300
67 Impala 327