using flash with either a Fuji GSW 690III or Mamiya 7II

B&W film (usually Neopan or Tri X) used exclusively.

I have a Nikon Speedlight SB800 which has an auto setting (just have to set the ISO and aperture)

I usually use the Mamiya in aperture priority mode. If I use the camera with the flash (in order to decrease shutter speeds and freeze action), how do I pick a shutter speed when thinking about adequate exposure.

Conversely, using the Fuji along with a Voightlander VCII meter, how do I adjust the shutter speed given a flash will be used? My aim in using the flash is to allow higher shutter speeds.

I think you mean increase shutter speed to freeze action (ie. 1/500th second for your leaf-shuttered cameras), right ? The shutter speed you use won't matter for your main subject (that is lit by the flash - as the flash will expose this correctly regardless of the shutter speed you are using - whilst using your SB800 on auto). The shutter speed will control how much ambient (non-flash) light is let in by the camera. With the Fuji, if you set the shutter speed to the one suggested by your meter for the f-stop you are using, this will be just like what your Mamiya does in Aperture Priority mode with flash (and without exposure compensation). If you want to raise your shutter speed to 1/500th to freeze action, this will be fine and the flash will expose your subject correctly, BUT the background may be dark (as the flash is not exposing this) - to mitigate this somewhat, use your lens at it's maximum aperture (f3.5 on Fuji, usually F4 I think on the Mamiya).

Thank you Damen. I don't always need 1/500 but can usually get away with 1/60 or so. This would let the background receive more ambient light. Presently, my shutter speeds are either 1/2 to 2 seconds, much too slow for what I need.