First time user / Help with engine/transmission combo please

[Steve Schmidt]

Hello all!
Steve here, this is my first post ever and I'm glad to be here! Outstanding link, tons of great information, nice work!!
I just purchased a 66 Chev C10 shortbox fleetside pickup truck yesterday. You see its a present for myself as I just retired from the Marine Corps after 20 years. I got her for a deal at $600 less motor, transmission, and fuel tank. This truck will be with me forever, I intend for her to be a steady daily driver with good gas milage (20 mpg or better) with a vintage air conditioner running as required here in this Texas heat.
My question, what recommendations do you have for an engine/transmission combo that will obtain my two goals of gas mileage and running an A/C unit? The truck originally came with a six cylinder, three on the tree, and I assume hwy gears in the rear. (not positive.....yet)
I can obtain a 250 straight six for around $1100 dollars but I believe it will have too much draw on it to run an A/C unit that is if I can find one that will work with a six cylinder. I also looked into a small block (305 or 350) with a 700R trans with overdrive. I don't believe I will get mileage better than maybe 14 mpg with that setup. I think the six cylinder is probably best, need help please .....