Re: D18 swap and rear axle
I already have the setup you're considering and it's great! '79 CJ-7, 360, T-18, Dana 18, Warn OD, and offset Q-trac AMC 20. Yes - just assemble all of the Dana 18 components into the Dana 20 case and bolt it up. Leave out the interlock pill between the shifters and you'll have the rear-wheel drive low-range you need with a front locker.
I LOVE - did you hear? L-O-V-E this setup and my friend Greg has the same thing except 258 and T-19, but still Dana 18 with offset AMC 20 and he loves it too. Having the pumpkins lined up is really nice off-road. The rear driveline angle is better on the 18, and the twin-stick setup is great off-road - especially with lockers. On top of everything else, you get 2.46 low-range instead of 2.03, and no, you won't need any extra gears, that's only for building a 2.46 Dana 20 which is not what you're after. I have the Warn OD on mine for long trips and that's also great - it's super as a gear splitter on steep mountain roads since the gears are so wide on a T-18.
We both run Q-trac CJ-7 AMC 20 rears with a welded on truss, welded tubes, one-piece axles, lock-rites, and 3.54 gears. Neither of us has ever had a problem with these, and they're easy to get, cheap to build up, and strong with the mods I've listed. Plus, they match right up to your front axle in width and gearing, and the offset is just right for the Dana 18.
Do it - it's great. - Chuck