I was in contact yesterday with one of the programmers who currently works at Canonical and had worked on the 304.137 driver before Nvidia declared it's end of life.
He suggested that the only course of action now would be to file bug reports against the Nouveau driver so that upstream developers can see it.
Hopefully get full Nouveau driver support for those earlier Nvidia cards that require the 304.
Here is an excerpt from our exchange:
I came across your name associated with your work on the Nvidia 304.137 driver. I would like to know if support for it has been dropped in Ubuntu 18.04? Or can it still be found and installed and would it work properly. I'm using Ubuntu Mate 17.10 and all works well with the 304.137. The nouveau driver that installed by default when testing 18.04 crashed the system repeatedly. Any insight into this is greatly appreciated. Thanks.
Unfortunately, NVIDIA dropped support for the 304 driver. You can file a bug report against the nouveau driver, better if on the freedesktop bugzilla, so that the upstream developers can see it.
So that's where it stands at the moment. Hopefully the Nouveau driver can be tweaked to function properly with Nvidia GeForce 6xxx and 7xxx GPUs.
Hope this helps anyone who's had issues with 18.04 and 304.137 driver...probably better to downgrade 16.10 LTS for now...and file your bug reports, hopefully they'll fix the Nouveau driver by 18.10.