Use an International T-19. They have the same gear ratios as a T-18, but have a synchro first (not that it really matters) You'll need 2 -
A)one from a Scout with a Dana 20 - typically this will have a 4:1 gear. From this you'll use the case, mainshaft, and adapter plate. You should be able to get one for $100 - $150.
B) One from a 2wd International truck - these will typically have a 6.32:1 gear. From this, you'll use the entire gearset except the input shaft which is also 4th gear. You should be able to get this for $100. They're everywhere.
C) Purchase a new 13-tooth input/4th gear and front collar from Border parts.
Dissassemble both trannies, and reassemble so you have the Scout tranny with the 6.32 gearset in it, and a Ford input shaft. You will then need to drill the bellhousing, and bolt the trans to it and install as a unit. Use the special pilot bushing from Advance adapters. The trans mount will be farther forward, you'll have to move the skidplate up a little from the stock 4-spd location and re-drill the frame or use some angle iron - no biggie...
I've done two of these, and a Ford T-18 swap as well, and the International route is the way to go. One final note, the Dana 20 that you get with the Scout tranny (almost always for free) is a great candidate for a twin-stick swap, which is something else every hard-core Jeep should have...
Finally, if you really feel like scrounging the junkyard to death, you can look for the ubiquitous Scout with the single speed chain drive transfer case. Because these have no low range, they all seem to have the 6.32 gearset in them. Then you can swap out the input gear for the Ford unit and you're done! I paid $125 for the last one of these I ran across...
The International stuff is everywhere, and cheap. When you use the Ford T-18 stuff, you seem to wind up at Advance Adapter's door - with your life savings in hand... Yes, I know my plan only works for Dana 20's (and Dana 18's with a little extra work), and not the Dana 300's, but with the 6.32 first gear, you'll have all the crawl ratio you need, and the 20's are just as strong or stronger than the 300's (no aluminum tailshaft housing and no strap-style u-joints... Plus, 300's hang too low for me. E-mail with more questions if you like - I've done the swaps and wheeled very hard with them.