Start here and learn of the variations of the T-18
http://www.novak-adapt.com/knowledge/t18_t19.htm
I thought I read somewhere that the T-18 from a J-truck had a long input shaft and deeper bell housing that the CJ application. You will want to keep total length in mind with a CJ5.
I looked into this once and decided that the NP435 was a better way to go. They are plentiful in junkyards (Ford trucks) and short enough for a CJ5. (Read better replacement parts availability)
http://www.novak-adapt.com/knowledge/np435.htm
The downside is you need a bellhousing for a T-150, T-18 or T-186 to mate to the 304. You will also need an expensive t-case adapter. Novak sells the custom pilot bushing for this swap as well.
This is what I was going to do untill I obtained a TH400. Now I'm regretting my decision as the TH400 is long as hell and I'm scratching my head trying to resolve my driveline.
No matter what trans you swap in, your driveshafts will have to be modified. The T-15 you have is the (IIRC) shortest factory installed in a Jeep. Also watch your rear drive shaft angle. IIRC, that factory 1310 single cardigan driveshaft can only sustain ~ 20* If you get too long, you're talking $$$$$$$ for a custom 1350 CV shaft and rotating your rear axle.
good luck