My friends 440 made quite a bit more power than it oughta... had pretty good top end too... that was a Holley 670 Street Avenger that i tuned for him. My 440 was the same way... junk that ran like a built engine... that was only a 750. That 650 with a solid tune should be fine, especially for what you're after and with what you have. When the time comes and you need more, if you're a tuner, you can get quite a bit more airflow out ov that 650 even. There are quite a few lil tricks that will net you more CFM and you can get into the 700's without buying a new body or carb... yet, you'll still have the small-carb driveability. My friend's 440 always did daily-drive a bit better than mine did.

I've always been a fan ov doing the most you can with the least. Anyone can dump on an 850DP and run, but the car will like the smaller carb almost every time, and in many cases run quicker anyways.

All that said, if you happen across a nice 750 i'd pick it up... but theres no rush to do so.