On its side

I need to put my boat on its side. Is the forestay enough to keep the mast in its base or do I need to put the jib on and tighten it up.

If your shrouds are tight it should keep enough pressure on the forestay that you won't need to raise your jib.

IF you are worried about it, you can always used the jib haryard to tighten the forestay with out having the jib attached.

If the forestay is pinned in the lowest hole in the shackle and the shrouds are also pinned reasonably low, then that should be plenty safe.

I've done it lot's of times.

Or... tie a line through the downhaul cleat, halyard and jib halyard cleats to the striker post. Tied tightly, that would hold the mast in the step cup even if the shrouds were loose.