carb tuner
see similar MPG on my heavy 400SBC Caprice and the
carb mixture is dead-nuts by the wideband. With no
overdrive and no EGR, this may just be what you have
to live with. It's also not out of line with other heavy
big-V8 vehicles I've had in the past (455 Olds 98,
390 Cadillac, 350 van, 383 Chrysler Newport, are a
few and none of them ran over 10MPG).
First step is probably just to put a wideband sensor
up the tailpipe and see if there's any need to change
mixture.
Could be that you want to raise the power enrichment
kick-in rather than messing with base mixture.
pete that builds my old engines is in palm bay, he may be able to help you, let me know, ill get you his number
there is also TJ off ellis road that has a shop with a dyno, called "trickpro"



