Flightline is OK for a mainsheet, but the polyolefin cover will probably degrade faster than you want. It'll get "prickly" and stiffer instead of fuzzing up and getting softer like polyester / Dyneema blends. 9.5 mm (3/8") is a little large. 8 mm will run through the blocks better.
New England Bzzz Line is a better call, and at $0.72 / ft for 8mm, a lot cheaper than Flightline. New England Salsa is also a good call; still cheaper at $1.39 / ft for 8 mm. I've used both and they've held up well - gotten better with use in fact.
Harken 351s are too big for the jib sheet if you want to go low profile. Harken 224 Micro Blocks are what come with new boats:
32' x 1/4" is the size of the stock jib sheet.
