I'm considering converting an old Polaroid SX-70 foldable camera to shoot 600 film. But would that make the old electronic flash I have for the camera useless because it's meant for use with SX-70 film? Would it overexpose all the photos I take with the camera?
Just buy the ND filter Polaroid sells for the SX-70. That's what I use when I shoot 600-series film in my SX-70. It slides into the top of the film pack. I use a Mint flash with my SX-70 with no problems.
Yes, a simple meter adjustment to allow the camera to correctly expose 600 film will increase the guide number of flashes by 2x. The simplest way to correct this would be to put a 2 stop neutral density on the flash after converting the camera. It's inefficient, but those old full-power-only flashes were always inefficient.