72 Chevy K20 Transmission Swap?

[RaptorSentry]

I have a '72 K20 with the turbo 350 automatic transmission. I am just curious right now more than anything, but is there another transmission with possibly more gears that would fit in between the engine and the transfer case, without any major modification. I'm not willing at this point to move the engine or the transfer case. My goal is to improve fuel economy, just not smart enough on transmission to know what is swappable.