Body Reassembly Questions

[72MARIO]

I am going to start putting the pieces back together so that I can prep them for primer and know that they are going to line up when I go to install them. I was going to start with placing the cab on the frame then installing the front clip. The main reason I am doing this is to check the body lines My question is should I assemble the box and put it on too? Or take the chance for some fluke reason once everything is painted it wont line up right (that would require getting a LWB floor and cutting it to size cuz I cant find a shorty anywhere!) I think the answer is getting more clear as I keep typing. The reason I am doing this is that all the body pannels have never been installed on this cab before. Any advice would be great on how to put everything back together!