Before you switch carbs would suggest the following:
(A) do a tune-up
(B) with a friend who is GOOD with a stop watch, go run about (3) quarter-miles and average the times.
(C) fill up at the same station, same pump, facing same direction at run about 3 tanks of gasoline through vehicle making a note of the mileage traveled.
You now have somewhat of a baseline for performance and economy to compare after you change carbs. You may be in for a major surprise.
Don't know much about the Demon other than what I have seen written (some good, some very bad).
Holleys are cheap and parts are everywhere. Expect to buy many of them to dial in the unit on your engine.
Cannot answer if the carb will fit your manifold; but you can. Four barrel carbs are basically spread-bore (ie Q-Jet) or square-bore (ie Carter AFB). So if your Q-Jet fits the manifold with no adapter, you need a spread-bore carb.
Do not use a spread-bore to square-bore adapter (or the other direction either).
As to brand, pick one that you (or your engine tuner) is either VERY familiar with, or plans to become very familiar with; as there are no perfect carbs out of the box (although the Carter TQ comes close).
Let us know which way you go, and the results of your comparison tests.
Jon.
