I don't think you can (or want to) mess with the sheave box, the Isomat masthead wouldn't lend itself to that.
Isomat does make a spinnaker crane that bolts right on -- it attaches with the same bolts that hold the "lid" on the masthead. The crane presents eyes to hang blocks. I am not 100% sure, but I believe that the lead was fair from the blocks to the existing outside masthead sheaves. The blocks just hang there to clear the chafe at the masthead and to get a bit more clearance from the furler. You can see the cranes here ->
http://rigrite.com/Spars/Isomat_Spar...in_cranes.html
I think you can clear up your problem in an afternoon and for not too many dollars.
Bob