The original 68 rally's were 14x6 your using the 69 and later 14x7s which you can pick out a mile away by the trim ring so no need to worry about rwl or redlines for shows. to me you defiantly need wider rubber then the stock 68 tire size or they will look to small and skinny on those rims. There are a lot of threads on this site with photos having to do with tire sizing. As far as putting the oe shackles back in that car with the old springs that will be a crap shoot because there were only 2 reasons people used those: 1 hopefully for you, the car sat at the correct height but the owner wanted that 70s jacked way up look. or 2 and usually the case the old springs were dragging arse and it was a cheap way to raise it up. the easiest way to check this without going through the work of switching shackles is to look at the arch of the springs. If they have a pretty good arch or are almost completely flat. If not worn and really flat but sit to low for your liking with the oe shackles and you still don't want to replace just cut the extender shackles you have to whatever height you want and re drill. By the way just so you know radials have no bearing on outer edge wear. it's driving habits, aggressive cornering or if your putting a lot of miles on or not.