If it's just the radius arm itself, then you can get access to the "inside" nut with a socket + extension bar (about 3" ?) to clear the bottom fold of the chassis. For the other end you can either secure it with a ring spanner jammed against the body, or if that keeps coming loose, some mole (vise) grips will stop it moving.
If it's the actual rubber mount you're trying for, then the same process but it's easier with the radius arm out of the way, either completely removed or just tied up out of the way. Then you can tack weld nuts on the backing plate once you've got it in pieces so it's easier next time. (it's a Lotus, there will be a next time.....
)
Brian