building a long distance touring car

Stock 1969 Belvidere (4 door) equiped with A/C.
318 with torqueflite
Timing perhaps slightly advanced to take advantage of the higher octane fuel sold by American
21 mpg on family trip from North Jersey to West Fla. when the car was relatively new.
So I'd say that should be your baseline for both milage and pep.
With an A-body and 4 spd, a stock 318 build will get at least that.
Radial tires will help pick up perhaps another 1 mpg.

'69 smog efforts were mostly focused on idle and decel.
Set yours up with maybe 5* more initial at idle, and a hair richer for a little better fuel and power at idle.
Let the intake dictate whether you go spreadbore or squarebore. That assumes you don't want to go factory 2bbl.

Bottom line. You should be able to get at least 22 mpg in the coastal plain and piedmont with a teen build and still have plenty of snap on the street.