One more difference in the 4L80e is that they are all computer controlled, there is not 4L80.
The TH400 was an option in your '73. I'm not aware of an HD version of the TH400, but there is a TH475 that is a little stouter and has support for an output shaft mounted emergency brake. The gearing in that TH475 is a little different if I remember correctly.
I would wonder if the valving would be different in a transmission setup for a Diesel. The torque band is lower and if the transmission was tailored for that motor it may seem to shift too soon behind a gas powered V-8.
I must admit that while I own a couple of them I don't really know a lot about them.
