What Tom said. I'm doing this project right now. I'm naive and sorta frugal and picked up sale-priced Dyneema SK-75 in 6mm. Warpspeed is covered (looks & feels like double braid line) and much nicer. A horse hairs' width of this high tech stuff is probably strong enough, but your hands will want at least 1/4" or 6mm. For the sheaves: I found matching the width more difficult than other dimensions, so I ended up with sheaves smaller in diameter than the originals that probably violate whatever the recommended line-size-to-sheave-diameter rule of thumb is. Next size up was too big. I got the little white delrin ones at Svendsen's, about $3.50 (x4). Steve Seals probably has something better. Cake project if your mast is down.

:: 1/4 warpspeed,

:: Steve Seals or Svendsens has sheaves,

:: Easy to do if mast is down.
::
::
::
:: :: Hi,
::
:: :: I need to replace my main halyard. NOW. It is currently
:: :: wire and I want to switch to line. I have a few
:: :: questions. What line should I be using? Is a replacement
:: :: sheave easy to find?
::
:: :: Lastly, is this a reasonably easy DIY project?
::
:: :: Thanks in advance for the advice.