66 Impala V8? Chances are excellent that it came with the 327 especially if is a true SS, although could have come with a 283, slimmer chances that it came with a 396 Powerglide like a Caprice that I had from Grandma, 100% stock. A true SS could have been a Muncie M21 4 speed, I don't think an M22, or could have been a Powerglide 2 speed automatic, will have a center console between the front buckets. '66 was my second favorite next to the '68. I had a '68, factory 396 and last year of manual transmission in full sized, M21 in my case. For their size, both remarkably light especially for BBs to motivate. Wish I had it back. Stupid, stupid, stupid me. I've done a lot of stupid things in my lifetime, but not being perfect makes life more interesting.
Good luck in your decision.