FWIW, I've seen information that suggests it may be possible to permanently alter an SX-70 SLR type camera to expose correctly on 600/779 film without a filter. There's supposed to be a capacitor that can be replaced with one of a smaller value, causing the shutter to close sooner (as it should with the faster film).
Unfortunately, this requires a) opening the electronics housing around the lens of the camera (much harder on a sonar model), and b) unsoldering and replacing a component on a circuit board with another that must still fit inside the reassembled camera.
Polaroid's suggestion, at least, doesn't require major modification of the camera (though finding a 1- or 2-stop ND filter to fit an SX-70 could be a little too much fun).
For good or ill, my SX-70 seems to have died; I found recently that I left a film pack in it for about 10 years, and even a newer pack with a known good battery (operates a 600 camera, in which I bought it) doesn't wake up the SX-70. I'll try it next time I have a fresher pack, just in case it needs more power than the 600 (P&S), but if it's corroded internally, it's almost certainly not worth fixing compared to getting an SLR-680 to replace it...