Aingh, that seems typical, since a new carb costs about 400 clams... they will try to talk you out of a new carb just do they can "rebuild" your existing unit, only to make money. I guess everyone needs to stay in buisiness, but I don't care much for those who say you need more than you really do in fact need.
Holley carbs are designed in such a way that even very young and inexperienced folk can service them, Grab a Dave Vizard book from your local speed shop, and you will be amazed at how simple Holley carbs are. I love them simply because should the need arise, a Phillips, a 5/16 socket, a 1/2 socket and a big flathead will allow me to operate on that carb anywhere, anytime, anyplace. and anyhow; even on the side of the road should I decide the setup isn't quite what I want.
As stated above, 750 is theoritically "too big", but if you have plans for building a deep-breathing engine, then the overkill of the 750 on your current combo is easily justified. Otherwise, many people just think that bigger is better when it comes to carbs, which is certainly not the case.
Alright, I will quit rambling now, but honestly, the Holley you have probably just needs some adjustment, not replacement. Go to your local speed shop, tell the service fellow about your situation, and request a "trick kit". It is the cheapest and best way to go if you want to fart around. Should you desire any extra cool stuff, get a "jet kit" which gives you countless Holley jets for a fantastic price ($50-ish) so you can tweak your carb all the way. Oh yeah, I said i would quit rambling, so I shall, my bad...