750 Holley or 650 Holley DP carb for 340-4 speed?

Really splitting hairs here imo? I could make all 3 of them work on that combo. If you really want a DP, and it "will" make your midrange feel stronger, I'd just go with a 750. That size is just too easily used on many combos you may change to over the years. I'm old school, so I'd just grab a List #4779 and be done with it, but there are some fancier models out there now days.....if you want to spend the extra coin?