I've got 3/8" lines.
I was planning on trying out single handing with the jam cleats anyway. I might run the line around the winch and then back to the stern cleats if the jam cleats don't work.
FWIW, here's a picture of where the jam cleat is located.
What is the line going through the clutch back to the cleat?
I see a cam cleat next to the clutch; to me that seems the logical choice for cleating off a sheet, but I need to study the boat more; I will go look at some pics online. However, based on your pic, It doesn't look like you would be able to simply replace the jam cleat with the Spinlock piece unless you installed a wedge base to lift the aft end of the Spinlock cleat. Otherwise, it does not appear that there will be enough travel downward to lock the Spinlock cleat....
My brain is unclouding, and I recall that Hunter jib management is cabin rooftop, away from the helm; I understand a need for a solution...
****EDIT****
I have looked at a few 260 examples online. Now I understand that the wall of the cabin trunk is just aft of the cleat in the pic you attached. To me, you would have a workable solution with the PX or PXR cleats
IF you were to simply add them as far aft on the cabin roof top as possible, maybe making a lopsided triangle defined by the jam cleat, the winch and the Spinlock cleat.
What you will want to avoid is accidentally locking your jib sheet in the cam cleat up there forward next to the clutch. If that happened, you would release the Spinlock cleat and the jib sheet would not be free.... Were you to try the Spinlock cleats, you might somehow remove or disable the cam cleat...
Other than that, I see that the PX or PXR cleats can be of benefit when single-handing your boat, being mindful of what I shared regarding line sizing. You might want to size-down to 5/16" line for your sheets if you can find line that will work for your load, (which I believe you can). It will be a little rougher on your hands, being smaller diameter, but it will move easier through the cleat than the 3/8" sheet(s).