Re: Santana 525 Main Halyard issue
Using a strippable core dependent line (not all are strippable) is a good option, just expensive. You'd be looking at something like Warpspeed.
I'd do 5/32" Amsteel to 5/16" double braid. It'll be easier than 1/8" Amsteel to 5/16" double braid. 5/32" should still fit over the sheaves.
Since the mast step has a hinge (I just checked the user manual) this will be an easy mast to drop. I really do think that it's worth dropping it and checking over all mast mounted hardware on a new boat. You can do the whole thing (drop, inspect, replace halyard, raise) in a few hours.
You don't have a location, is this boat in Seattle?
1986 Pearson 28-2 "Elena"