Izotope RX7 Music Rebalancer
Single best tool I've experienced.
In hardware, a Dolby 740 can do a lot but it's a very processed sound if used heavily.
Phase inversion and mono summing/filtering/re-mixing in various incarnations, as said.
Back before there was good digital noise reduction, I found with a lot of mono tapes with high hiss levels that I could transfer them on a stereo deck, and then use mid-side processing to turn off the side signal, which would be mainly hiss. Now with RX7 etc there are lots of tools for that.