For my MC1 I am using a 425mm Syncros post. With my saddle at 85cm from BB it's just enough. Only thing is that, in my opinion, I have to tighten the seatclamp very hard just to fit the seatpost properly. I have measured the post and seattube and it should be ok. Only Shannon makes 500mm seatposts but they are very rare and expensive.
The later MC1 and all MC2 use 31,6 wich just sounds and looks more save. Have had Team MC2 in 22" but had to make the decission wich to keep (you know, the money thing). Fortunately I have chosen the MC1 to keep. Or the later Rascals, they have a shorter part of the seattube on top. They seem to crack less harder then the older ones with the hughe part of seattube on top.
My first Attitude (20" Dolomite/team USA) had the seatclamp cut off and used a seatclamp. It was way too small for me but I dared to ride it as it should be.
Well enough said about the cracking thing, more about the meet..