Here's an easy way to use a hoist to launch your 420. Made with 1/8 in 7x19 wire. The hook on the longer leg attaches to the eyestrap on the inside of the transom (in the center down by the inside deck). The hooks on the two shorter legs attach to shackles you will need to add to your shroud adjusters. You will be replacing the lower in clevis pin that attaches your shroud adjuster to the boat with the shackle and shackle clevis pin from a Harken H138 D-shackle. You will be left with a shackle (facing aft) attached to the boat at deck level.
Shackles sold separately. Two D shackles are needed to make a complete system.