Isn't this the boat with frames spaced to hold a six packs snugly in the bilge bays? My memory is that the construction of these boats is utterly fantastic. Absent rot or damage, she ought to handle the tension you desire. That's the hull. You'll also want to be sure the stick can take the compression and the shrouds keep it in column.
If so and if you can measure the backstay tension on the hard beat where you have jib sag, that will give an idea of what's happening. I'd try tensioning to about that static and take another sail to see if you're leaking or hurting anything.
You may also want to consider a dynamic approach to rig tension like easing on the run and for sitting at the mooring, tensioning only for the beat.
That's the limited seat of the pants view. In WWI, each fighter off the assembly line had to be test flown to find the problems and correct them. Test flying those crates was incredibly risky and when my grandfather actually lived through a week of doing it, they made him permanent, cancelling his chance to get out of England and over to hunt the Red Baron. Grandfather said it was simple. Other test pilots took the plane up and shook it till something broke. Grandfather had the revolutionary idea to shake it till just befoe it broke.
Point is, there are too many inponderables with a old boat and old rig. I don't think you'll have much luck with predictive engineering, especially over the internet with no way to know the actual condition of the boat. But assuming you inspect your rigging so there won't be some stupid but catastrophic failure, like the backstay turnbuckle stud snapping, then you are safe to gently tweek tighter depending on wind and observe closely.
G'luck
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