H16 Mainsheet Blocks

Lower block:

Harken 2629 BLK-57MM CARBO TRPL RAT.W/CAM ($221). You'll need to add an eyestrap above the cleat to serve as a becket.

Alternate lower block:

Harken 194 BLK-2.25 IN. LP TRPL HEX W/CAM ($161)

Upper Block:

Harken 2640 BLK-40MM CARBO TRIPLE ($82)

The combination of these blocks gives you the lightest weight (19.5 oz) and lowest stack height of any blocks on the market.

The worlds boats in Mexico had Harken 194's and the 2640 40mm Carbo blocks on the boom. I like the Carbo Ratchamatics because they're lighter (all of 3 oz) and the self-releasing ratchet mechanism lets the mainsheet pay out much more easily, especially in tacks and in light air.

I should have also asked, 3/8 line but is there a choice here too?

3/8" is easier on the hands, but a lot of racers use 5/16" since it runs through the blocks better.

Have to tell you that it is hard to beat the Harken Ratchamatic, if you can legally incorporate it into your class. Ratchet while you are sheeting, and when you ease it is free wheeling. I could go on and on about how great that is -- but for example, most blown tacks are because the mainsheet won't ease enough in light air. With the Ratchamatic, the mainsheet eases very well.., this eases the leach, allowing the boat to go through the turn.
In other words, the system works.
I am not sure what is allowed in the H16, but take a look at what is available:
Here is the link to some really good systems:
http://store.catsailor.com/tek9.asp?pg=products&grp=331

On the line, the modern stuff (i.e., Warpspeed)) the core is all the strength. And the core is UV protected. So, you can get comfy cover and the part that goes through the blocks can be the core -- definitely the way to go.
In general, 3/8" is a bit too large.
Rick