6'3" and 260 is going to be tight in the M cockpit. I would also want to have the ballast in when he was moving about on deck. Standing headroom in the cabin at 6'3" is going to be at the open hatch area only. He'll have to bend some everywhere else. Having said that, I believe it is the most headroom of the Mac boats.
The head in the M is near the front of the boat. This is a low headroom area, and toileting needs to be addressed from a seated position (at least that's the rule on my boat). Which brings us to the choice of fixture. Without being indelicate, at 6'3"' 260, there is a lot of accoutrement to direct into a small opening with the typical 2 gallon porta potti.
@DrJudyB went through the search for the perfect fixture not too long ago and could probably shed some light on this issue.
The fact that you want to sail and pull a tube does put you in the power-sailer market.
Picky is good.