Beam will defo interchange between them, just note the different roll bars used on rear, same shape but thicker 14mm on cossie, not sure on your size, but 4i here was 10mm, even the 14mm one was not good enough to be called useless even!!
I managed to get a LT77(rover) box in the tunnel ,and they are quite bulky boxes certainly much bigger than T5, I modded the mount using the one I had and the rover gearbox mount together took about an hour, so no biggee.
tabetha