One thing I've noticed on my 2001 PT425 with the factory seat and armrests is that I can get to a full turn in a place with great undulations in the ground and the rear of the armrests will hit the front of the engine compartment cover. Don't know if the geometry is different on the newer design PT425s or not. Just be wary of that if you decide to build a cab. Run it through all the possible turns lock-to-lock and run the front right tire up on a block while running the left rear up on a block to make it fully oscillate while fully turned to the right and left. And put the seat all the way back, too. You can then measure the clearances.
Also, its a possible pinch point, if, for some reason, you stick your fingers or hand back there while turning and going over uneven terrain.