Carlton,
in my first attempt, I plonked the mast in the position where it is easiest to fit it. However, there is also a question of how the mast should fit with respect to the interior. From a technical point of view, the mast can be fitted as far aft as through the hatch, or even maybe further aft, if you go for a hi-balance Split JR. The mast may also be offset a foot to one side without ruining the sailing. Remember, without shrouds, offsetting the mast is technically fully possible (..but will probably generate a lot of questions from by-standers...).
Soo, take a good look at the boat with the mast position in mind and then tell us what you think.
Cheers,
Arne