is it possible to put the sm465 into a 79 cj5 with a 258 and a dana 20? i think that there should be some kind of adaptors for that swap, but from what i hear those can get pricey. also, would it be that much of an upgrade from the 3 speed that i have now?

Sure is. you will want to do some research first. The 465 had 3 different outputs ,10 spline early 4wd models which should work with your 20.They also had 32 and 35 spline outputs. Your 20 is most likely a 10 spline(possible it's a 6 spline). http://www.novak-adapt.com/index.htm Lots of good info here. You would have 6.5-1 first gear versus about 3-1. Rear driveshaft length should be fine. Dont forget about adapting it to your engine.

Go to the Novac web sight they have a world of info on trans. swaps, and yes the 465 is hell of an improvement.