I don't know what size the sheaves are for a 35-III, but I'll share that the sheaves on a 32-III are 2.5" diameter, 0.475" wide and nominally sized for 3/8" line.
All my current halyards are 3/8", all rope. I've got a spin halyard that is telling me it wants some attention this year, and I'm probably going to replace it with 5/16" VPC so that it runs free with minimal resistance from masthead or exit slot.
My main and jib halyards will likely stay 3/8"... jib only goes up and down a few times a season, the rest of the time it is furled; the main halyard stoppers are sized for the 3/8" line and it ain't broke, so no need to change it.
$.02
Bruce
ETA: photo of 32-III masthead with 3/8" halyards. I think I have a photo of one of the sheaves, too, if that's of value...