1972 GMC C2500 Other applications

I would like to replace the granny gear tranny with something that has a better ratio such as a 5 speed or 4 speed. What other transmissions would work from other truck/cars that would work better. Right now I have about almost 400 horses under the hood 350 with a I think muncie 4 speed with low gear. I have 411 gears in rear and am changing to 14 bolt axle with 342 gears. Would it be wise to keep the tranny I got now with the 14 bolt or upgrade to something more geared for the high way and not so much for towing. I was once told a 5 speed tranny from a chevy truck might fit but need a definite answer.