There's no better mainsheet block than the Harken Carbo Single. Laser Performance suggests you use these 40mm blocks as the two blocks that hang from the boom on your 420 by a 90 degree head. Made of high strength resin, the Carbo is lightweight, strong, reliable and affordable. What more could you ask for?
Pair with LP91340 and LP91283 for a complete mainsheet system.
What's that? Looking to upgrade even further? Check out APS's Mainsheet Bridle, APS918.
This part is also listed as a H2659.