You are happy that the jib (or genoa) halliard is straightforward, and your query really concerns only the main halliard.
First point, the smaller diameter for the main halliard will be because the load is far less. The role of the main halliard is only to haul the sail up, and tension the luff, for which the tension required is only relatively modest. The genoa (or jib) halliard has to effectively provide serious rig tension, in order to keep the genoa luff as straight as can be achieved despite the distributed lateral loads on it; that is why you have a Highfield lever to tension that halliard, but not the main halliard.
Second point, that upper sheave looks to me to be sized for a rope halliard, which would have been standard at the time the mast was built. Most probably 3/4-inch (circumference) pre-stretched terylene at that time, equivalent to 6 mm (diameter) pre-stretched polyester today; however you may find that 5 mm renders through the sheaves more readily, and particularly so if any of the sheaves have been replaced by more modern ones. If you do choose to go for rope you may prefer 6 mm for comfort in hauling it up, but there is not much to choose in that regard, and if you were thinking of 6 mm it would be wise to try a test piece first to be certain whether it will render through the sheaves.
While you are about the job it is also worth checking that the sheaves still rotate freely; if they do rotate, lubricate them; if they don’t, then unless you can free them off you will need to replace them.
That apart, you have three options for the main halliard, of which I personally would not choose the second option (the type you currently have):
- All rope. That should work perfectly well, and if you decide to use the boat for cruising and wish to fit a reefing facility that is probably the best choice. If you use either pre-stretched or low-stretch polyester, stretch is very unlikely to be a problem, especially as the load is only fairly modest, but if you were nonetheless concerned about that you could opt for rope with a dyneema core. You can make this off to the same cleat that you are currently using.
- A fully spliced composite, as you currently have. That requires a rope to wire splice, which is a fairly skilled bit of ropework, but there are professionals who will do it. However I suspect that this would be more expensive than all-rope (but I have not checked the point), and with modern rope materials I don’t see any advantage in this type of composite; its historical justification was as a low-stretch alternative to the limited choice of all-rope halliards that were available fifty years ago. I also feel that a splice of that nature is inherently likely to be the weakest part of the halliard, but because the load on the main halliard is not particularly large this should still be good enough. This type of composite halliard (unlike the type you have for your genoa halliard) is a valid alternative if you wish to fit a reefing facility.
- A wire halliard terminating in a long “soft” eye-splice (i.e. non-rigid, not using a thimble) with then a rope tail attached by an interlocking soft eyesplice, just like you have for the genoa halliard. The wire loop then hooks over a toothed rack, in place of your existing cleat. You would need to obtain and fit the toothed rack, and it is most usually fitted to the side of the mast. This type of halliard should not be used if you intend to fit a reefing facility, as it would then require the wire-to-rope join to come under working load, which it is not designed to do, and which would be an unfair load for this construction.
For wire halliards, in either case, go for the most flexible type, 7 x 19 construction. (By contrast, use 1 x 19 for standing rigging; it is stronger, but much less flexible.) The original material when this mast was new may well have been galvanised; that was a popular option at the time, but has largely fallen out of favour today, although a few owners still prefer it. Personally I would go for stainless, because it lasts longer and requires less maintenance, but its lifespan is still not indefinite; expect 10 years maximum, and periodically inspect for broken strands at the splices and also anywhere the wire becomes kinked. If you find any broken strands at all expect others to follow, so replace the wire.
Hope this helps,
Oliver