454 + TH400 + 4.11 POSI = no freeway

[Phorbes]

Just picked up my first C10. It's a gorgeous truck, black in color, long bed and in all around excellent condition. Previous owner lived downtown so he had no use for freeway driving and I'm a little outside of town and would certainly like to be able to drive on the freeway.

Long story short, I took the truck home (on the freeway) and I could barely make myself go above 60MPH because the motor was SCREAMING!

I know it's not damaging anything, just burning fuel. I want this truck to be cruiser, to help pick up supplies at home depot and haul the dogs around in. Not a show truck or a drag racing truck, just a good all around usable truck.

Right now I have the standard 265/70/15 tire sizes and I know I could put much larger (in diameter) tires on to help bring down the RPM. There's no Tach on the dash so I'm not sure what kind of RPM I'm at right now but I would have to guess 60MPH is at least 3k+

Question is, what do I do?

Do I take out the rear end and put lower gears? I know the POSI is excellent and bulletproof and worth a pretty penny so I'm sad to do that.

Should I just slap on some super large tires on the rear?

Should I get a different tranny with more gears? TH400 runs good, gear shifting is a little sloppy, but I'm not sure if that's normal or concerning? I have an 86 C10 I had purchased to work on and its sitting in my garage that has a TH350 on it but I believe both of them are just 3 speed transmissions so that really would't help to swap would it?

I don't mind spending another $1000-$1500 to make it work. I'm new to the old truck world or even carbureted motors. I'm a BMW guy and have always wanted an old American work horse.

So that's that, what should I do?

Thanks for the help in advance!!!