How can I improve gas mileage on my 1975 Chevy C-10 pickup tuck with a chevy 350 crate motor in it

That is going to be difficult.Your driving habits are about the best thing you can do, some mods to the engine will help ,cam headers, intake and crab, rear gears but are kinda spendy. but you will be lucky to get much better than 16 or 17MPG no matter what you do.

Go to a 2 barrel carb

Try to change the way you drive. Other than that you can clean motor parts, change oil.