Going from MVA to ported isn't going to effect fuel economy assuming the ported source is correctly located. Using ported vacuum simply means that you aren't adding VA timing at idle and coasting.
I use and prefer ported vacuum for most of my engine builds because they are higher compression with well chosen cams and will not like, want or need a butt-ton of timing at idle. Most are fine with 10-14 degrees initial timing and adding another 10-15 degrees via the VA does NOTHING for them aside from making me close the throttle plates too far at idle and not as steady in and out of gear.
Anyhow, the extra fuel you are consuming has to be going someplace. Assuming the distributor is getting the job done that just leaves the carburetor, and those units are not well known for being super efficient for "normal" driving. I've spent some time with a few Holley and Holley clones, there will be significant improvements waiting for you with custom tuning. You'll need to go into the metering block and air bleeds to clean it up, changing jets and PV's will be a complete waste of time/funds........IMHO........Cliff
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https://cliffshighperformance.com/
73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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