Ya, your '68 will have a D18 tcase, single lever.
A T14 tranny was stock
Front axle was a D27. A later model D30 will practically bolt in place, but the caster will need to be adjusted to get 5 - 7° positive.
Back in 2000, I installed a '74 D30 in my '71, replacing the stock D27. I kept the drum brakes, but drilled the drums for improved wet & dry performance. Worked great!
I added 5° steel "shims" to adjust the caster to a target value of six degrees positive.
Lots of tech related to your '68 can be found at EarlyCJ5.net... the best source of info on the web for early CJ5s.