Bolt it directly into one ton of concrete just as RA Lister recommended,
Do NOT rubber mount it and certainly do NOT mount it on a railway sleeper.
RA Lister stipulated solid concrete bases for all there generating plant that was to be run for an extended period for a very good reason. Flexible mounts means broken exhausts, failed electrics and work hardened copper wiring that will snap.
Rubber/flex/wooden mounts are only for in boats where there is no other option, mains failure sets that run infrequently or shipping containers and other moveable structures.
Don't believe me, just check out the manuals, OK, I know there will now be a deluge of folk telling me how 'damn fine splendid' there flexible mounts are but I know a guy in his sixties that served his time with RA Lister in Scotland before branching out on his own. All he does is go around remote highland estates and islands fixing and installing Lister generator sets and he's been doing it for forty years.
He once told me that his 'bread and butter' was fixing wiring problems caused by flexible mounts and railway sleepers.
Think about it chaps, why would they specify tons of concrete if a few rubber mounts would do the job.
This is my old ST2 running with a pound coin on the rocker cover, loosen any one of the mounting bolts just one quarter of a turn and it will fall over.
I see yours is for a standby set Frank so you may well get away with rubber, but long term you're just asking for trouble. Remember these engines were designed to run 24/7 and that's what they'll do if mounted correctly.
Cheers, Paul