I have two thoughts.
First is that by first cutting the two cabs above and below the belt line, below on the one you want the top from, and above on the cab you are using, you can make access to those bolts and welds easier.
If the reason for cutting up these cabs is because the lower cowl is bad, you might try a different approach. If you cut the firewall and floor from the good cab to replace the bad parts of the five window you are rejoining only a few inches on each side of surfaces that show. To join the cabs in the cowl area is a lot less trouble than that joint across the rear.
I've done some cab cutting too, and its a lot of work. I've not put one back together... yet.
