A 43 ft Gulfstar masthead sloop with 1set of non-swept spreaders, forward and aft lower shrouds and, I assume, cap shrouds with chainplates abreast or very slightly aft of the mast base?
In most cases, the mast on such a rig would be tuned with no prebend and 0-1 degree of rake.
Adding a furling mainsail on a extrusion aft of the mast would affect the balance of the sail plan. Mainsails on headsail furlers are not common, to say the least. This rig is a “one off” and not part of the original design. You’ll have to experiment with rake to get the helm balanced.
Id start with with zero rake, and then test sail it in typical conditions with the working headsail. Adjust the length of the forestay to increase or reduce rake until the helm feels balanced when using the working sails.
Mast prebend isn’t going to make any difference to the shape of the mainsail, so iI wouldn’t tune any into the rig unless you see the mast pumping. The forward and aft lower shrouds should suffice to prevent pumping.
A mainsail for use on a headsail furler presents some design problems because the furler extrusion will have a sagging “hollow” curve in it when the mainsail is under load. Usually, a standard mainsail is designed with some amount of positive luff curve, which contributes to the position and depth of the draft. A headsail luff is always cut with a hollow, not a positive curve, to compensate for the sag in the extrusion. Mainsail performance is affected by changing the curve of the luff track, so I imagine the performance of the mainsail will vary with wind loads and furling.
To be honest, I think that using a headsail furler for a mainsail is a very big compromise in sail shape and control for a way to achieve convenience. There’s a reason why none of the spar makers or sail lofts offer such a product line.
Good luck.
On edit - fixed a few typos and grammar.
