To get a synchronized low gear you need to step up to a T19 and they are scarcer than the T-18 .
Check this out:
http://www.jeeptech.com/convtrans/cjt18/
I swapped the
T-150 in my
CJ5 for a
T-18 and never looked back. It was an easy swap, direct bolt in. It will shorten your drive shaft slightly. I used 1"x1" rectangular steel tubing to space the skid plate down to get rid of driveshaft vibration. This was how it was done back then. 1st gear is 6.32.
I used to drive my
CJ5 to the trail all the time. I drove from SoCal to the Rubicon many times with the
T-18 , 4:11s, and 33" tires. No problems. I loved that set up.
The shortest 4-speed is the SM420 with a 7.05 low gear. I run this now in my Chevy swap. A very balky shifting
Transmission . The
T-18 is a really nice shifting
Transmission by comparison.
If your considering an x-fer case swap the 1980 short-shaft
Dana 300 would be good; although they are very hard to find as the short shaft was used only in 1980. The short shaft version is really nice for CJ5s because it doesn't add to an even shorter drive shaft.
I measured for an NV 4500 swap and I found it too long. Either need to move engine forward or rear axle back or both. I don't believe I've ever seen an NV 4500 swap in a
CJ5 .
The TH350 will leave you with a very short drive shaft. If you ever plan on running a Dana 60 rear - plan on moving the axle back and the engine forward. Also the 1996-1999 4L60E is actually slightly shorter than a TH350 and it has the advantages of and
Overdrive and 3.0 first gear over the TH350s 2.52. You can swap a 2.75 into the TH350. With machining I'm told that you can use 3.0 out of 700R4.
For a nice simple manual
Transmission swap the
T-18 is the way I would go. I miss my old
T-18 .