Tom,
I?ve done this. It was a bear, but it can be done. I did it by making the F-404 assembly into two sub-assemblies. One set of F-404A uprights was one assembly, the lower F-404 D and the other uprights were the second sub-assembly. They had to be riveted together after they were in place. This required some back riveting, but it can be done. In order to remove the old F-404 assembly, I had to first remove one set of F-404A uprights also. I didn't see any other way to accomplish this.
I built a 4 that had been started by another builder. They didn?t keep the F-404 assembly properly positioned when riveting it into position during fuselage construction. The gap for the main spar shifted slightly making it too narrow for the spar. The bolt holes had also shifted. I had to purchase all new parts for the F-404 assembly and match drill them to the existing holes in the main spars. This required setting up the completed wings in my living room (my wife still talks about this) with the proper dihederal, clamping the new parts to the spars, then match drilling each hole. It was a big job. It took me 160 hours to complete the repair. If your new F-404 bulkhead and wing kit (spar) are already drilled, it should not be as difficult for you.
Mine is an early kit (#68). The early RV-4 kits didn?t come with these components drilled, it was up to the builder. At some point Van?s started drilling these at the factory. My guess is that they built a drill fixture and drilled them on a drill press but that?s pure speculation on my part.
A call to Van?s could be made to find out if they feel a different spar may line up well enough to be used with the existing bulkhead assembly. It states in the construction manual that even the assemblies drilled at the factory should not be considered interchangeable. They emphasize how critical this assembly is.
Be certain to make a spacer the same thickness as the spar and keep it bolted in place in the F-404 assembly when you rivet your new assembly into the fuselage. Hardware store bolts are good enough for this. If the first builder of my kit would have done this, I wouldn?t have had to remake the F-404 assembly.
Good luck.
Cameron Smith
RV4 #68