I always lose money with buy/sell transactions. Switching 67-72 trucks will cost you a year's fuel costs, at least, unless you upgrade to a meticulously maintained, 6-cyl truck which would cost even more. These 30+ year-old trucks rarely come to us in perfect shape. I have spent over $1000 on each of my two 4x4s, so far. Worn brakes, old radiator & hoses, carb rebuild, fuel tank issues, etc add up quick. Not to mention oft-neglected interior work. Maybe you could do a better buy/sell by buying a truck that isn't worth near what yours is. And auto transmissions lose major mileage behind little 6 cyl engines. Maybe a lockout tc gives ok mileage at cruise. A short-throw, Muncie 4-speed would do better.
For the money saved sticking with your truck, you could closed-loop fuel inject the BB. Next rebuild, add hyd roller lifters and a cam made for the lifters and the EFI computer's capability. You should gain some mileage, though maybe not with enough savings to justify the cost for a decade or so. Since towing is not an issue, you might look at higher gearing, too. And how much of your mileage is due to how long it has been since the carb's last rebuild? At least add a vacuum guage to gauge your own driving habits. Lots of savings often available there.
Now, if you're talking about a second truck.........?
