They're air shift.Can these transfer cases be swapped into the M39 series of 5 tons without too much difficulty? Assuming they are air shaft rather than sprag type?
The ratios aren't the same. It really isn't worth mating up the Tcase out of the M939series with the transmission out of the M809series as you end up with a gear or two that aren't useful.
I'm thinking the M39 series is the same as the M809series except for the engine. Correct me if I'm wrong.
I'm guessing that your potential motivation to do so would be to ditch the sprag in favor of air shift. My opinion is that losing the robustness of the T138 to get the air shift of the T1138 isn't really an upgrade, even before you figure in final drive ratios. (I own both 939 and 809 trucks. I have no "mine is better" bias.)
Wandering off topic, my experience has been that the 809series trucks perform pretty well even without lockers. I got one stuck in soft mud and buried it to the axles. The sprag worked like it was supposed to, and it was spinning all 10 wheels. I did finally get it free under its own power.
A word or two on 16.00's. They noticeably rob bottom end power. To the point that it effects driveability. This is less of an issue in 809series trucks than in 939series trucks. It's like every gear is one gear higher than before.. I have 16.00s on my M925 and end up using the Allison auto like a manual transmission. Running around on back roads at 45mph i end up holding it in 4th gear and really only see 5th for the interstate or higher speed highways. If your truck actually works for a living you may want to consider 395's. 395's are considerably cheaper than 16.00s, and don't take nearly as much away from the bottom end. If you're planning on doing low speed and heavy, 395's are worth a look.