Old RV3
Congratulations,
I am in the process of converting a 29 year old RV3 to the ?B? wing. Because some of the early ones had certain builder irregularities in them I opted to replace everything forward of the seat back except the top longerons (which are one piece from the tail), the seat ribs, mixer box aft bulkhead and the motor mount. Since my motor mount was one of the old 4 bolt units, I added the two additional pickup points ala current motor mounts as sold by Van?s. I also replaced the two aft fuselage bulkheads, the horizontal stab attach points, the bottom tail skin and the stinger to accept a full swivel tailwheel.
If you can get away with just putting in the new center section and aft carry through and then just plugging in the new wings, you may find it much less complicated. I will take and forward any pictures you think might help.