"I feel like the 5 ft would be a better match to the tractor when cutter is in the up position on the 3 point--When mowing, a 6 ft would be fine and when you raised it, the front of the tractor could get a little light"
I'm running a 5' cutter through some pretty thick brush (initial cut after a year of wet weather and no maintenance). That's with a Deere 670 which has 16 HP @ the PTO. Although I'm running in low gear, even going through very thick stuff (tall too), I've experienced minimal bogging down. Haven't stalled the machine yet either.
That 4310 should handle a 6 footer with ease if my little machine can handle a 5" cutter.
When I transport, I just raise the cutter enough to get the front a bit higher, then drive with the tail wheel (of the cutter) on the ground. Now, when I raise the cutter completely off the ground (going down a road for a few hundred feet, for example), I can feel the front end of the tractor getting lighter but no jumping up and down. This is with the FEL installed, driving at a reasonable speed. I'm sure the front of the tractor would bob a lot more without the FEL.
And, of course, another alternative is to get one of the tow behind cutters that hook to the tractor draw bar. It's my understanding one can go to a larger sized cutter with a tow behind unit. Down side is the potential for the rear tires of the tractor to rub against the front of the cutter in tight turning. That's an operator awareness issue.