tire swap question on farmtrac 270

[Ryan03]

I have a 270 farmtrac with R-4 industrial tires, , it looks like the Montana 2740 is the sister model to my tractor, and it appears to have larger front and rear tires. I have a lot of mud and slopes on my property, and could definately benefit from larger tires. My first Question is what size front and rear R-4s come on a 2740. My next question is does anyone know if a ag type tread is available in a flotation type profile similar to a R-4 . My current stock tire sizes are 23-8.50x12 front R4 and rear 12-80-18 R-4 rear