lots of for and against the two fitty 3 but they are a neat tidy small v8 that sounds good and can run cheap. the later blue motors with 4 barrel are terrible (except for the ign). the early two barrels ran quite economically. we had a servo years ago and looked after a fleet of fords and holdens for an a/c mob. The hq 253's in the wagons and pvans would turn in 23 - 27 mpg while the 250 fords would do 18 - 22 mpg.
the real answer here is big block crate motor, why naff about? but for a budget build, 253 is as good as any and easy to find...ive got one out the back paddock!