Got me a new Dart Swinger

Some may (and probably will) argue the point, but if you're going to keep the factory 340 exhaust manifolds, you probably won't see much or any difference going to the Air Gap. The factory intake & exhaust are an exceptionally well balanced system when used together. To use the Air Gap without a corresponding increase in exhaust efficiency probably won't get much for net gain. Assuming you're running factory J heads, I'd be considering a set of Eddy or TF heads to take advantage of those 416 cubic inches. Get the Js worked on as a bare minimum- they can be made to flow really well, but for what that will cost you're in the ballpark of a set of aluminum heads. Once your breathing capability is increased, then look at installing the AG and headers.
As far as the Holley 750, are we talking vacuum secondaries or a double pumper? And which Eddy carb are you currently running?
Since the car is new to you, I'd personally run it as is for a while before tearing into it, just to see a little more clearly what you like and don't like about it, and then make plans for some changes to address them.
Your HP ratings, I assume, are 490 crank HP and 380 rwhp? That seems like a lot of driveline loss...