Matt,Now that I have had a chance to drive the car some, I have found that I need to check the electric choke operation, carburetor jetting, and possibly tweak the mixture / idle screws. I experienced some stalling / hesitation from a cold start. Once the car warmed up everything was great and the "seat of the pants" test still indicates some extra power out on the highway at speed. This Sprint manifold was treated with a heat shielding coating and that may be affecting the current "tune" as the under hood heat dynamics have changed.
I'll be very interested in learning what you have to change in the carb settings, as I have the same problem since changing out to a Sprint manifold, and don't know what I should try first. Although it feels like I have increased power, my gas mileage has fallen way off (although some of that could be attributed to a little more "lead" in my foot as I "test" the car out), from 18mpg just after adjusting my valves a couple months ago, to 13.5mpg on my last tank. I also have a lot more problems when cold, like the engine wanting to die when coming to a stop light (only in the first few minutes of running), hesitation on acceleration, slight backfiring on deceleration (more of just a kind of rumbling than actual backfiring, and I actually kind of like the sound). One other change I made in the last few weeks was adding a Magnaflow 11365 2 and 1/4" muffler (no resonator tips yet), but the headpipe is still the stock 1 and 5/8" diameter (using a new Bosal unit courtesy our friend Bob Legere).