It's a trade off with 3 speed automatics; great off the line or great highway cruising manners. Personally I'd do a 3:23; a big block should smoke the tires off the line regardless, plus this will give you liveable 70mph RPMs. Of course your best bet would be a 700R4 with a set of 3:73s but if you're keeping the TH400, id go this route. Great car, I like them early 70's Impalas