I think your cam is killing your MPG. If it makes power from 3500-6500, your probably never in its power band. Going from 3:23s to 2:76 will probably make it worse because you getting farther from your power band.

Doc Fiberglass is a Thermoquad fan. Personally, I think you would just as well with an AFB. I have not heard a Projection was any harder on gas then anything else, I simply think they are a bit less tunable then the newer efi stuff is, which gives them a bad rep to some. You already have the projection, I'd keep using it.

Swap the cam for something more street friendly (talk to a cam grinder) and change to a 2:76 gear, and see where your at. I think 14-15 mpg with city and country driving, on todays gas, would be about as good as I would expect with a 383. When its all done, the driver has as much effect on fuel mileage as any other part of the car. You have to constantly drive for fuel mileage if you expect to get it. Gene