Sounds to be a fairly major mod. The result won't fly like either model, so you will need to start with reduced throws on everything for a start. As to where the spar will be in relation to the CG that won't change much from where it has always been. CG position is a function of the wing MAC with a few other variables thrown in but for most models it is in the 25~33% MAC range so no matter where you put the wing, the cg will always be in the same place on the wing (unless you intend to put the wing behind the stabiliser or some other unorthodox position), and therefore in the same relationship to the spar.
Evan, WB #12.