In deference to the OP, Olsen911, I'd like to share how we reworked the back end for the "S/T" flares...
We started out with, stock rear quarters, "RS" FRP bumpers and ducktail on our car.
The "S/T" flares were 9" wide, so the impending issue was how to "blend" the existing "RS" bumper with the "S/T" flares.
If you decide to get a wider "S/T" bumper, your problem is solved.
Can you "massage" a narrow "RS" bumper to fit the wider "S/T" flares?... absolutely.
And here's the final assembly...
At this point, it becomes so easy to say, "...this will do..."... RESIST that urge.
A smooth transition can be had... just spend a little more time being fussy about it.
