SBC performance

I have a stock remanufactured 1978 305sbc. I added a Holley 600cfm (vac. sec.), a Edelbrock Performer intake and a set of Hooker Headers. The timing was set at 14 BTDC with a total advance of 44 BTDC using stock HEI. The vehicle was getting 12.5MPG consistently.
In the attemt to increase mileasge and performance, I swapped in a Holley 390cfm and a new HEI distributor with a 50000 volt coil and new wires and plugs, of course.
After adjusting everything and running the vehicle for a few weeks, the mileage has actually "DROPPED" to 10.7MPG.
OK-now-shoot!!! Tell me whats wrong!!

Kick your timing back, and do a vacuum test on it (leaks)
Check your vacuum when you set your timing. Set Highest reading. Rich

Some 4 bbr carbs have smaller primarys than a 2 bbr carb.
600 cfm/4= 150 each, 390 cfm/2=195 each, maybe even more
so with a spreadbore type.With the 4 bbr,check your plugs and
rejet you may have left something on the table.

Paul

What gearing are you running, and what kind of car is it in??