This was mentioned a couple of weeks ago about leaving out RU7 and just increasing RU5 to maintain the track.
http://www.haynes.co.uk/forums/showt...t=rear+upright
Personally i think your upright will be stronger for having RU7 their,also it gives you more surface area to weld to to strengthen RU8.One thing i would also bear in mind is that the parts were all designed and tested on CAD and you may well be creating something not as strong as the original,unless you have the means to test it.