289 cam change

I have a 65 Mustang with a 289, 270H Magnum cam, poorly matched to a Stock C4 and 2.80 rear gears. I am planning on changing the cam over the summmer, but would like to ask for advice from people who have tried different cams. Since it can get complicated, I want you to know that my focus is NOT horsepower, I'm going for gas mileage and good low end torque, maybe a step higher than the factory cam, which I was happy with. I would appreciate any recommendations on a good size and type cam for a 289. I've heard that the newer Xtreme Energy cams from Comp Cams are good, but I'm kind of worried about the quicker opening and closing of the valves. By the way, don't be afraid to get technical, I've done my homework on overlap, duration, centerline, lift, etc. and know how a cam works. Last, I have both the factory 2 BBL carburetor and manifold, and the 4 BBL Carter 625 cfm carburetor and Edelbrock manifold that I'm currently using. Would I be better off using the old 2 BBL with my smaller cam, as the 625 cfm is really big for a 289, or would it work o.k.? Thanks in advance for the help, guys.