To my knowledge, other than auto's, the only 2 transmissions that came in '76 to '79 cj's were the t150 and t18.
Right now you have a T150 tranny with Dana 20 case.
Option 1: T18 - They did come in some CJ's but are hard to find and expensive. Other common thing to use is a t18 out of a cherokee or larger jeep. You have to change the input shaft and modify your current bellhousing. Without a adapter you have to run the stock dana 20 transfer case. Unless you find a super rare scout dana 300 that will bolt up to a t18...or so I'm told. This option can you get a 4 speed with a granny low but your ratio in the highest gear will be the same as what you have now 1:1. But remember not all t18's have granny low (6:32:1 I think).
In 1980/1981 they switched gears and did away with the v8 and dana 20 case and everything came with a dana 300.
The T5 has a nice over drive gear. It could be either .7 something or .8 something. I have seen both. The negative is that T4 and T5 are considered light duty versus your current t150 which is considered medium duty. I run a t5 behind my 304 and have for several years. People will tell you that it can't happen and depending on how you drive it can be easily broken. I will probably break mine but I have a spare. I also like the later tranny options or adapters so you can run a dana 300 which is a much better transfer case IMO. It has a much lower low range of 2.62:1 over the dana 20 2:1.
Now the bellhousing of any AMC engine should bolt up to your 304 but the clutch is a different story and may require using parts from both a 3 speed and 4/5 speed kit. Also the skid plates are different and will have to be modified. Goes without saying you will probably have to cut new driveshafts.
Just a guess but I'd say about any route is going to be expensive. Between a tranny, case, clutch, shafts, etc. I wouldnt be surpised if a swap would be between $750 and $1500.