I have always been weary of gripshifts, but that is because is probably because of cheap sram versions that come on BSO's. I used to work in a bike shop and kids used to destroy these shifters very quickly, not durable at all and also feel horrible to shift.
Not just that but it's also awkward to shift and brake at the same time.
I mounted my rapidfire shifter in a manner that's reachable on my Nitto bar exactly like your, will take a quick snap tomorrow to give you an idea of how to mount it.
alternately, bar end shifter mounted on thumbie is always a solution (expensive though, keep looking out for them in jumble sales, found an 8 speed one for £15).
Not just that but it's also awkward to shift and brake at the same time.
I mounted my rapidfire shifter in a manner that's reachable on my Nitto bar exactly like your, will take a quick snap tomorrow to give you an idea of how to mount it.
alternately, bar end shifter mounted on thumbie is always a solution (expensive though, keep looking out for them in jumble sales, found an 8 speed one for £15).