2 speed 6-1, 3-1 purchase MAINSHEET VANG.
Nine alloy sheaves with Torlon ball bearings.

Pull both ropes to bring the main sail in quickly or fine tune with either rope. 
Use continuous 10mm line (joined with a splice or a knot)  (rope not included).

How to measure how much line is required:
First, fit the blocks to the traveller and the boom
Add the next three lengths of rope together to determine how much you need.
1. Weave a short piece of rope through the blocks as per how they arrive, this determines the block rope length for your vessel
2. Measure from the cam cleats to the shrouds and double this measurement
3. Add another metre for good measure

Here is a picture of a 40-2sp vang on a 38ft yacht. It is attached to an MT2TP Traveller with a heavy duty spring.

NOTE: To maintain complete control, use one rope to ease the mainsheet under load, rather than two.