Or simply sell that R6 shock to someone else and get an XT600 rear shock. Perfect length, just the lower bushing needs either replacing with the XV750 bushing or just a little bit extra as it's only 2mm larger than the XV one.
If you want to raise the rear get a pre-89 rear shock, if you want stock height get a later one. Spring rate is perfect for one person and acceptable for two-up-riding. (And yes, I speak from personal experience...)