First C10! 1975 4.1 inline 6

[Natl128705]

Getting ready to buy my first C10, been wanting one for a while and finally found one that's a decent price and actually runs! I've had chevy trucks before but nothing anywhere close to this old. This one has a 4.1 inline 6 with a 3 speed, was just wondering what everyone thought of that motor, I'll probably keep it for a while at least since it is already running, What upgrades does everyone recommend? Anything to help gas mileage? I'm thinking already I need to put a different transmission in it, great torque, but man she's screaming if I take her out on the highway. Recommendations, thoughts?