Jeff, I have experienced this as well. I broke second gear twice under the strain of the V8. No more V8. As far as popping out of gear...Yes. My experience on two seperate vehicles / transmissions was due to the shift fork wear. You can't tell just by looking at the fork because the wear is uniform, it gets to a point where it does not fully engage the shift collar, therefore when I let off the gas it pops out of gear. Not a real big deal, but annoying. I changed out the forks each time and the problem was fixed. Finding good forks is the hard part. Luckily the that problem has not occured in my current trans / vehicle.
Regarding the trans swap...No bolt ups that i'm aware of, but Anything can be made to fit.
Steve