Will SBP discs from a 71 work on a 66?

Yup, just get everything off the '71 A-body. You can keep your '66 control arms but everything else from the lower ball joint out, has to go. You may have to swap the brake master cylinder push rod if it's a manual system as I seem to remember there was a slight length difference. Use the '71 lines and proportioning valve too. You'll love the improvement.
Mark