I think the compartment of a 6hp motor would still accommodate the 5.5 cylinder head. This motor (the 5.5) actually started up, albeit I just threw some gas in to see if it would pop. Runs well, but it's not pumping water, so the powerhead would need to be yanked anyway to change the impeller. Might as well slap it on a 6hp pan and leg if I'm already yanking the powerhead. I'll save the next guy the grief of having to pull the powerhead just to change the impeller.
I read the HP rating for the 5.5 and 6.
The 5.5 says 3500-4500RPM rating operating range, 5.5hp rating at 4000RPM.
The 6 says 4000-5000RPM operating range, 6hp rating at 4500.
I guess my only sticking point is, would it be necessary to change rods. I don't think that it would, but would love to hear the 'panels' weight in on this one. I am thinking the carbs are the same for both motors, less a style difference for the choke and needles. But the change to needle bearings at the cranks wound up squeezing an extra 1/2 hp. So doing the powerhead swap out without changing rods would still yield a 5.5hp motor, since the bearings would remain the same.
Not worth spending the extra 2 hours swapping bearings just for another 1/2 hp...I'm not sure how you could tell a difference anyway.
"switched by popular demand from my animated prop!"