I did the swap, It was not very straight forward, in heinsight i may not do it again and stay with the 545.
I picked up an MT643 from Eli at Diesel Core with all the needed parts, he currently has one for sale with all the flywheel wheel, bell housing and flexplate.
Issues I ran in to,
1) 643 bell housing was too wide, I had to get it shaved at a prescision metal work place.
2) 643 starter position was on the opposite side. Rewire starter and main buss bar connections.
3) Speed sensor port was open, had a challenge finding a plug for the threaded hole as my speed sensor is / was on the front wheel drum.
4) Dip stick tube position different, had to bend the tube to get it to kind of fit.
5) Piping to existing oil cooler (factory oil / water radiator) was too small for the oil flow from a 643, had to buy and install a derale hi efficiencey cooling rad and fan, all new piping.
6) The trans is 200lbs heaver, it will add extra weight to you 5.9 that it was not designed for, in our situation the extra weight it lifting the front of the engine on its mount and vibrating through the chassis.
7) could not get it in the gap between the front axle and engine, had to tip it up on its end to get it up and over the front axle,

needed to shorten the drive shaft by 1.25" as 643 is a little longer
Although I do have locking now in 3rd and 4th, no real gain in MPG, no change on top speed as the 643 has NO overdrive so 4th is still 1:1, same as the 545