The W-2 rocker arms use a off set arm and one regular arm, same as the 340 T/A head. Most everybody carries them. Crane left overs if possible, Comp, Huges, Harland Sharp, etc....
I quickly found these Harland Sharp Rockers. There top notch brand name units. Also, remember, the W-2 came in versions that have rocker arm pedistals for a quick and easy rocker arm mounting and one without rocker stands so you can correct the geometry.
These are pricey units;
http://www.summitracing.com/parts/CSP-S70026WEK/
http://www.summitracing.com/parts/CSP-S70025W2K/
The smaller chamber would be a (very) closed chamber vs the open natural state of a X J etc.. 340/360 head. The differnce is 72 vs. 55 cc's. Use this difference in a compresion cal. and whatch comprsion jump. It is often said that 1 point rise is approx. 3%. Of course, the huskier build will develop more power as a milder build willl (IMO) not gain as much from the rise from the extra squezze.