Our rules for an F14 (as a completely new class/design but by incorporation of existing 14'cats by encouraging their inclusion by useing a "grandfathering" rule) allows any length of mast you like, but you will find that you soon reach the maximum practical mast height some where about 26' after that the rig becomes imbalanced, in fact I personally feel that around 25'6' is as high as you want to go and still have an all round balance to the boat, others may differ but thats why there is no restriction on the mast height so that experiment and innovation can occur while still remaining within the "box rule" of the total sail area that can be used. As far as mast step position is concerned, the best balance is at or behind the centre of the hulls,fore and aft, if you place it forward of centre then only place it as small an amount as you have to as the further forward it is the more pressure is put on the bows while sailing and the more prone the boat is to pitch pole.
Darryl