I'm guessing you don't want to go for a fitting so make sure you get "taper tip" shafts. (.355). They are available in Nippon shafts. The rest . . . you're on your own for.

I would go with the reshaft instead of adding butt end extenders. The NS Pro 990GH is not listed in the Titleist Custom Options booklet, so you will need to find a good repair shop/person and they should be able to reshaft very easily.

The link below is for the Custom Options booklet:

media.titleist.com/.../

I'm wondering if you are thinking of the GH 950 shaft which is a 95 gram taper tip shaft. The standard shaft for the 712 AP2 was the True temper Dynamic Gold which was a heavy weight lower launch and spin shaft for high ball speed players. The lighter shaft would lower the swing weight somewhere between 2.8 and 3.8 swing weight points. If this is your situation and you wanted to use the the lighter shaft 1/2 inch added weight will add approximately 3 swing weight points and two grams of head weight adds approximately 1 swing weight point. In order to get there clubs to their intended swing weight obviously isn't simply, however, a good custom shop should be able sort out the issues and come up with the best solution for you

You will find most of us on here beleve in being fit buy a quality fitter. But if you decide to extend them you will change the swing weight quite a bit making the heads much lighter feeling.

Just want to note that each added 1/2 inch will add 3 swing weight points giving increased head weight feel- experiment with extending one club with qualified fitter to see what fits and proceed to a shaft change if needed- there will be many shaft options in the weight range of the currently installed shaft.

I think the 990GH was a shaft cally featured as a Uniflex shaft and is probably a .370 shaft and if correct you would have to bore out the hosel in order to fit.

I guess it depends on how short they are. Adding butt extenders isn't hard, but neither is rehafting. Reshafting may require some ferrule turning which is somewhat of an art itself... adding butt extensions isn't hard and all you'd have to do is regrip... that's only if you like the currently installed shafts... if not then go with the reshaft, but make sure you get the right ones so you don't have to revisit the whole project.