Re: 700R into the CJ6 behind 304....and a D20 ques
The 700R4 is slightly longer than the TH-400, but how are you going to get that into a CJ-6!
Better have that rear axle kicked back or the drive shaft angles are going to be brutal and the slip yoke is going to be longer than the drive shaft!
Anyway, the 700R4 is only slightly weaker than the TH 400 and that is only in shock loading.
You will want the intermeadate sprague changed out to the larger, stronger style.
I'm with the other guy, I'd run a Big ole' tranny cooler in it, and I'd consder an electric fan in front to help things out.
When you are wheeling you won't build any more heat than any other automatic in the lower gears, but heat is the killer of autos, so it's 6 one way, half a dozen the other...