THE best mileage for SBC is the old Crossfire injection system from 1982-84.
Next would be an electric Q-jet setup w/700R4.
For what you have think BLOCK HEATER and use it faithfully. If you never cold-start and run on choke pumping cold oil with a daily driver your actual mile goes way up. You can also go with synthetic ATF in all the gear cases.
Have the front and rear ends checked for square with the frame, 'dog walking' scrubs off mileage.
Have fun with it.