The other comments are spot on, just one additional thing. Check the integrity of the side stays before trusting the full load to them.
Also - you said it is on the slip, so the nose is up ? If it looks like a steep slipway you could use some ropes from where the spreaders join the mast and run them forward to another strong point and give it a bit of tension.
The people working at the slipway should also be able to provide some guidance.
We re-rigged an Adams35 out on the mooring by just using the side stays, it works. We took the forestay and backstay off and took them ashore to the rigger. Then the lowers next trip, then the uppers & cap on the last run. I would not do this in a bay with ferry wash though