67 Chevy C10 Custom Flatbed

[Phoax]

There is a 67 Flatbed I've been thinking about buying for a couple months now and I have a question I would like to ask you experts.

I test drove it a couple times and it has a 3 speed manual with a 327 that is at way too high rpm at highway speed for me. It has a floor shifter but there are remnants of a tree shifter which leads me to believe it was originally 3 on the tree.

I really want to put a 4 or 5 speed manual in it. I do not want an automatic. What is my best option for a manual that will give me better mileage on the highway. I read about a 4 speed that bolts up but the top gear isn't any wider.
I don't plan to do any towing, just enthusiastic driving.

I don't mind some welding but if I have to re do the whole drivetrain I probably won't buy it.