I converted mine by finding a slant six pump and input shaft and I went with a manual shift valve body.
A 999 will probably have the low gear set and possibly the 5 clutch drum with a beveled stator and high gear drum. If you want to use the beveled 5 clutch drum in a non lockup application, you will have to have the bevel custom machined onto the non lockup slant six stator. Or your other option is to use a 4 clutch drum with thinner frictions to get 5 clutches.
The other area of concern is again with the stator. If you use the forward clutch hub from the 999 with the slant 6 stator, the sealing ring bore diameter needs to be machine bigger by .060 thou.
If machining is not available, then you could also use the slant six forward clutch with the slant six stator, but your clutch count will be less because of the snap ring location.
Do not use a non lockup forward clutch with a lockup stator or you will have a no drive situation because of the difference in sealing ring diameter(.060) mentioned. Be sure your end plays are within spec.
