so i'm rebuilding my original 1970 340

You get 22 mpg now, how much more can you possible get and what really $$$$ savings would that get you, what do you spend on gas a year now $500, $1000, $2000? even if you could save 10% on $2000 is only $200 a year. If you do a lot of Highway install an OD tranny but I wouldn't neuter the performance too much for a couple more mpg's.