Joe - for what it's worth, I tried both the OS and ST NVAs on various OS engines, including some LAs.
On my engines, the OS worked MUCH better and I did not have to ream the case.
I'm not a fan of the ST NVA - I prefer the positive "ratchet" type as in the OS and ENYA.
When you install it, have the hole in the spray bar turned slightly off center. The 4 to 5 o'clock position is where the pressure is lowest (suction is maximum). There was an interesting article regarding this in an older magazine.
Bob Z.