Advice on 77 Scottsdale

[MNiceGuy]

I've been using this message forum for a little while now but this is the first time I've decided to post something up. I don't have the knowledge or mechanical skills that most of you have but I've been having a lot of fun lately fixing up a '93 C2500.

I also own my dad's old '77 Scottsdale 1/2-ton 2WD. I am a little ashamed to admit it's been sitting in a shed for at least 5 years with little to no interaction from me. I feel it's time to...well...you-know-what or get off the pot when it comes to this thing. First off I'll share some pics:





The interior is completely original. The exterior has had a lot of the sheet metal replaced (i.e. no bondo) during a restoration over 10 years ago.

The engine is a 305 with a 2-barrel. There's about 180K on the clock but absolutely no oil burns or leaks. Pretty amazing really. This is mated to a 3-spd manual column-shift transmission.

The thing I can't get around is what to do about the transmission. This is where hopefully someone here can give me a little wisdom. The transmission was never meant for today's speed limits. At 70-75 mph I am running fairly high RPM and getting the gas mileage of a small aircraft carrier. At first I was looking into changing to an automatic but lately I've been wondering about a 5-spd out of a 88-era truck.

Can someone tell me what would be involved with swapping the transmission? Would it be easier to go automatic or manual? What are some options I should be looking at?