500 AVS2 or 650 AVS2 ???

As some of you know, I've been throwing carbs at my 65 Dart 318 for a couple of years. Have a 600 Eddy on there right now, not bad but looking for a change (don't ask me why). Long story short, looked at posts going back a few years about carb sizes for 318 and everyone's thoughts. It's a 318 with 230hp at the crank, about 180hp at the wheels, 8.25 3:23 posi. As some of you know we do a lot of cruising, long trips ( Route 66 a couple of years ago, awesome trip)
It's going to be AVS2 500 or 650 I really want the 500 for mpg but I'm leaning toward the 650 to make sure I have enough carb (never been over 4500rpm ) My question is, can I calibrate the 500 up or am I better calibrating the 650 down to be a little bit more fuel efficient? I pretty much know who's going to respond and I appreciate all of your guys input.
For what ever reason Summit just knocked $90 off there AVS2 650 so that's what has my asking, Tilly :)