fuel economy for 258

Well, a T-5 swap would be easier to find parts for. Same for six or V-8 (V-8 swap uses six bell).
But it does sound like you have tuning issues, especially with the carb. You should be able to easily get 15-16 mpg, which in your case would be a big improvement.

IMHO the exhaust is way too big for 258 inches, but that shouldn't affect the gas mileage that much. You could try restricting it with something like a 1.5"-1.75" exhaust tip in each pipe. I don't know how much that will affect gas mileage, but should improve cylinder scavenging (you need a bit of back pressure for that at low speeds!) and may improve low speed torque.  A 2.25" single is all I'd recommend on a 258, with 2.25" duals on a healthy street 360-401. Big exhaust really only helps on drag racers that need to breathe well over 4000 rpm. Street cars seldom see over 4000 (actually more like 3000...), and don't benefit from big exhaust systems.

Your carb choice is pretty good, going to a 2V shouldn't help -- once the carb is tuned for the engine.  You're only running on half that carb 75% of the time after all.

jdarosa wrote: Good to hear on the automatic. I drive it like an old man, so it's not from my foot.  I'll mess with the timing and get it tuned, and check for fuel leaks...I'd hope to get some mileage out of it before the big job of swapping in a 4 speed...oh lord. As far as the converter, it's non-locking and whatever the stock one would be for an 83 Concord. Thanks everyone

It's probably better that you don't know. After another guy that had issues with fuel shooting straight up out of the carb, I think you'd know what a "puking" power valve looks like. You would ultimately have an extremely rich run or otherwise completely flooded engine. Dieseling would be the least of problems there.

Have you checked out Ollie's American build thread??? 21 MPG on the highway, similar engine build. http://theamcforum.com/forum/ollies-66-american-convertible_topic70568_page1.html

One trick is to put a vacuum gauge in the car. and drive with the vacuum as high as possible.  Hook it up to manifold vacuum.   but if your cam doesnt let the vacuum be at least 18-20 idling, its going to be hard to drive the car with a gauge.  I did it in a chevy truck.  went from 9 to 12. but the cam killed low end, and the truck had bigger tires than stock.  and the carburator was not up to par either.    I have a stock 74 Gremlin with a 258 3spd.  Ive never really checked the gas mileage, but it gets about 16-19.  and I drive slow and not over 65.  your foot can really affect mileage too ..................... your automatic doesnt help mileage, but I believe with tuning, you can improve it.