I don’t know the safe limit,  but I would be inclined to play safe,  keeping the tensioning technology in period with the boat.   So any technique up to and including a Highfield lever should be OK,  but a muscle box or a cascade tackle might be risky,  and especially if tailed back to a 2:1 inside the boat.

In the early days of GRP boats,  which includes at least the earliest Bourne Plastics ones,  there were occasional stories of structural failure,  including chainplates pulling out of the boat.

Hope this helps,

Oliver