Since the Model 20 pinion is offset, can I still use the 9" from a F150 and leave the full width, or do I need to offset it too? I plan to extend the wheelbase by lengthning the frame the same as a CJ8. Its going in a 81' CJ5. with a RE 4.5" leaf spring lift, got new perches on order

That depends on the offset.

One thing to think about is why do you want to swap the M20 for a 9?

The M20 has a 8.8" ring gear in it, the 9 is 9". Put good shafts and maybe a truss on the M20 and you are pretty much equal to the 9", Unless of course you have big plans for 35 spline shafts and such in the 9"

Yea, big plans. Want to stretch,strengthen the frame and build a CJ8 running 35" or 38.5" tires. Any idea how long the body is on a Scrambler or a site I can find out? I'd like to put on a soft top this winter.
If I stretch the frame you think I can get by with the offset because of the longer driveshaft, thought about a double cv.
I think the 9" already has 31 spline.

I'm assuming you are running a quadratrac transfer case if so that will be your weakest link I'd look for a dana 300 or if the budget allows an atlas. This way you can center the drive shaft and have a better t-case in one shot. As for the 9in it will fit full width then you will want to find a full width 44 for the front. Or if you can find a full size wagoneer or J truck and grab the axles this would be f+r 44's for full width front poisen spyder makes a full width front axle swap kit to outboard your front springs.