It will be very tight if they do fit.
The 245's will have all the weight support you need.
I am quite happy with the M608z's, they are quiet and have excellent traction. They appear to be wearing a little quicker than I would have expected but time will tell.
Hauling weight with them is so much different than LRE tires, it's night and day. I love the way they handle and ride.
You feel the road a little more, but I cannot say it's really a rougher ride.
The heaviest I have been with them is 11,980 GVW with 7160 on the rear axle, 19,560 GCW. I really don't feel the weight, not like I could with 5500lbs on the LRE tires.