I did a couple of mods of the SX-70 Sonar to take 600 film because it's available locally at Sam's Club. I use the cameras to take kids' pictures at the Easter Egg Hunt at church. I evaluated the various methods (nd filter in front of lens, removing filter in front of light meter sensor, electronic mod) and decided on the electronic mod because: I wanted to be able to see through the lens clearly to focus, and I wanted a permanent fix. I also modded the springs in the film chamber so the 600 pack slides right in and out without a card. In other words, I ended up with a camera that's an SX-70 in every respect, modified to take the faster 600 film without ANY fiddling. It functions like a 680 or a 690, without the flash. You have to take the camera apart and replace a capacitor with one the right size and shape. I can even dial the light meter adjustment to tweak exposure if needed, but I haven't had to since the exposures have turned out just fine. If you want, I can e-mail you directions, or I'll be glad to modify one for you. I wish I had time to do a bunch of them, I could probably set up a shop, but I'm a pastor by calling and a tinkerer for fun.
--Rev. Ash