[spacemanspif]

I'm going to stand by my statement that headers MAY help MPG due to the fact that those old exhaust manifolds don't flow well at all and are definitely a choke point in the system. The Y-pipe before the CAT isn't the best either, nor the CAT itself.

You're running an electronic QuadraJet which has a love/hate relationship with car people as some swear by them and other swear they are only good for paper weights. Either way, they are basically the half step between carburetors and TBI because they are still carbs but can adjust fuel mixture like EFI. The carb isn't tunable quite like an old mechanical one but are still tune-able, just a little more complicated. A good book on Rochester carbs will show you where to tweak. One of the first steps is to make sure the giant secondaries aren't opening too soon and adjust your driving style to stay in the zone where only the small primaries are in use. Also turn the key to the run position and listen for the clicking of the mixture control solenoid it should click for about 5 seconds (Kinda like the fuel pump on new cars).

FWIW the carb on my Monte was never tweaked when I bought it with 140K and I didn't touch it either before I pulled the engine at 195k. But I also only got about 15mpg the entire time I drove it except for 1 summer where I ran it with a 2.29 rear on the highway getting 25mpg.

If you are set up like my 305 was there is a secondary air pump that pumps air down to the CAT. Located low on the passenger side if the engine. It should have it's own belt. If you want to fiddle with taking off ALL the other belts to remove that one, go for it, but I just cut mine out and never had any issues, removed all the equipment eventually but ran with no belt for quite a while. Those cars have a ton of vacuum lines too, a good check to make sure they are all in good shape and not leaking is one of those preventative things that keep the engine running well and might net a gain in MPG. The vinyl tubing that controls the HVAC ducts can get brittle and crack if you look at it wrong. Also check that the EGR valve is holding vacuum. If you don't have a vacuum pump, unhook the hose, reach under and push the diaphragm up, while holding it up put a finger over the vacuum port and let go of the diaphragm, hold for a minute or 2 and release; if you hear air hiss chances are the EGR is good. You can also keep a finger under the EGR to feel if the diaphragm is moving while keeping the finger over the port.

Sorry for the novel but basic maintenance is the key to a good running, efficient engine.