Daily driver small block build
i think mid-teens are achievable with a light throttle foot and might even tickle 20's on the open road if he's got some freeway flyer gears and keeps out of the 2ndaries on the TQ.
but city daily, probably 12
what sucks is that you can't easily go straight up fuel mizer status and find something like a SP2P or a 2bbl setup because of the magnum heads. the only way around that is money: drilling an intake and port matching it, or the EQ(?) heads that have both bolt patterns. or, again, EFI. but i don't see that returning massive gains over a well tuned carb set up.