One thing I think might be worth considering. How is the CG on your current vixen and would it be advantageous to have it more forward? I think, the Series 7 FWF moves the engine forward a bit. This will affect W/B.

I built two Model IVs with full gap seals and fairings on the empennage. It added a bit of weight at the aft 11 ft. arm. I found that my first one had a slightly aft CG which limited baggage capacity. I moved my engine forward 2-1/2 inches in the second airplane to compensate for that and it proved to be probably 1/2 inch too much - still within the envelope, but slightly more draggy as with nothing in the baggage sack in cruise, the elevator is slightly up from neutral.