Just a note since your changing things up on the t-case. First the 78 4-speed case is better due to it being stronger metal. You will lose some of your lower crawl speed due to the gearing is not as low as the 3-speed case. I hardly noticed a faster crawl in my granny gear and 1st gear 4 low since I have changed mine. The 4 speed case is bigger and while the mounting holes are the same on mine, I lost 2 inches of up travel on my drivelines (longer cone?

wider box?)and the rear drive line appears to be offset about 1 to 2 inches further to the passenger side. Had to retube the front(it needed it anyway-from candy striping) the rear is fine but the angle makes the CV joint at the case side closer and I worry about binding

.
Also the space between the body is closer(worse if you have a case saver on it). If I hadn't of done a 1 inch body lift it wouldn't have fit. My setup is different than yours. The first thing I had to do so the crawl box would fit(in 2003) is the body lift and that was with the 3-speed case. I have an sm420 to marlin crawl box to (new to me) 1972 4-speed rebuilt t-case. So all seems to be working well with the new case. Hope this might help when you go to install.
I have a transmission jack on wheels that I use so I have always loaded from underneath. Have done it 3 times. It is a real pain. Rusty has done it from the top which I think is easier but requires the hump out and some sort of apparatus

to hang it from. He is an engineer

and has the abilities to do that kind of stuff. Not me