I put a Muncie M20 in a 55 Chevy years ago and had a clicking in first gear. Turned out to be a chipped tooth in first gear. Just some food for thought.
Too, if your new one piece drive shaft is longer than 72 inches it's too long. I assume your truck is a long wheel base. If so, that's why the two piece shaft with center bearing support.
More food for thought. I worked for a Military Truck Manufacturer, again, years ago, and we had lots of problems with the deuce and a half rear drive shaft. Kept throwing them off. Turned out to be the weenie U bolt type U joints. After going to what's called 'Full Round' joints, problem solved. That being said I was in charge of the program, along with Rockwell, the manufacture of the shafts, to solve other problems. I did lots of research and learned a hell of a lot about drive shafts and U joints, which is why I know that 72 inches is crowding it. Too, with too long a drive shaft, you can hit what's called 'critical speed', which causes the drive shaft to look like a banana while tuning and eventually fly off. Had that happen a bunch on these trucks.
Keeping the U joints in phase, as the pictures show, and the shafts well balanced, is also very important.