How strong are they?

Real strong. gear drive.
Just an annorexic low range. 1.96:1

I was thinking that this would be a cheap swap for my heep because I have Ford D60's. I think that I would need an adapter plate behind my ax 15. Is the np 205 longer or shorter than a 231 with a sye?

I wouldn't mount that combination behind the AX15.
Adapters don't come cheap, and with the 60's and 205 the transmission is your next weak link.
Instead of buying an adapter, I would look for a better transmission that bolts up to your 205.
Sorry I cant tell you about differences in length.

The chevy 205 is about 12 inches in legnth now thats the fixed yoke version pre 80's I think. The 205 with the slip is even longer obivously...you have ford 60's so the only 205 that is good to you is the ford and some dodge versions were driver drop.
Why don't you mate a t-18 or Np435 with a 205 to the 4.0L and be done with it...It's a pretty simple swap. Search for some info on adapting ford trannys to jeep engines....The ax-15 is junk get rid of it......Sure you could probally sell th eax-15 and the np231 and do the swap and be in money if you are smart... :)

I forgot to mention, because of the 1.96:1 ratio from the T/C, another benefit of say going with an NP435 is the lower 1st gear, 6.69:1 compared to the AX15's 3.93:1. So you gain on your gear reduction instead of losing more.
BTW, pass up the 205's with the slip.