I agree with those who recommend EFI,,,, Those setups that constantly adjust themselves for driving conditions will likely yield the best results in A/F mixture.
A good set of aluminum heads will make a huge difference as well! I think I'll butt heads with the opinions on brands with some folks here, so I'll just say something different than an aluminum copy of the original cast design is what I'd look for! Modern combustion chambers should be studied while deciding....
An overdrive trans will help on the highways..... Keep the gear ratio in check as well.... The OD trans will let you experiment a bit on this one...
Roller camshaft, Roller rockers, roller timing set, windage tray, a good piston ring pack, light pistons, balanced rotating assy, electric fuel pump, electric fans will all help reduce mechanical loads on the engine as well...
A good set of radial tires that aren't super wide will reduce resistance, and lighter aluminum wheels will reduce rotating mass.
I agree with you,,,, the age of the block shouldn't dictate how we use technology available. I think taking advantage of every little thing and the attention to every detail will get you the 20MPG your looking for.
If it were me,,, I would dyno the engine and try different ignition systems, and settings. I would also want to try a few different intake manifolds with the EFI system to get peak performance at cruise RPM. Spark plug gap settings would be another thing I would play with while on the dyno.
My2Cents,,,,
