Waxman, let us know which thickness pad ends up working. Your project is essentially what I was planning on doing to my B3000, so I'll be very interested to hear about your results.
In addition, I'm trying to decide if I should add dynamat type stuff on the underside of the seat pan - not sure how much difference it'll make. I did find a product that supposedly has significantly less asphalt odor, since some of the dynamat products will off-gas for months in that small cab space. My plan was to install the mat on the underside (outside the cab) wherever possible. Also found a more dense foam isolation mat that I will probably use between the floor and rubber floor mat in place of the foam mat that Kubota provides.
I suspect your project will net some pretty impressive results. Only reason I haven't started on mine yet is that we are probably not done with the massive spring snow storms, but now that I've seen your dissasembly pics, I can start ordering all the components. I definitely like the idea of that rubber vibration isolation mat. :thumbsup: