I've done the swap on my cj. I have a '76 cj7 that came with a 304 and a t150 3 speed. In order to do the swap you will need a t5 bell, t5 tranny, d300, modify/drop skid plate and new driveshafts. Also the clutch is tricky. Have to mix some parts to get it to work.
I love mine. I hated the 3 spd off road. The d300 is lower ratio 2.62 vs 2.0 of a Dana 20. Tranny is weaker but I have a backup. The lower 1st gear, closer ratio gears, better highway gear and the option for a hurst shifter makes it a cool option.
Hope that helps.