I think your math is generally right. Now, all of my 40/45’s are running newly rebuilt engines including three from Mosley. They do not mind RPMs at all. The trucks I have with the 4-speed and 3.70 are just better to drive. They have a nice shift pattern and can cruise at 70 mph and around 2500...
The one on the left has the H55 5 speed and 4.10 gears. The one on the right has the Toyota 4 speed and 3.70 gears. Both trucks drive easily 70 mph +. However, I greatly prefer driving the 4 speed. Taking the 5 speed out of the 45 series will be a future project.