Reviving this to bring some closure to the frame clearance issue. From what I gather above and from my searches on other posts, the following is how much people have reported each header protrudes BELOW the frame on the 818, disregarding motor mount selection:

Killer B - 2" below
Perrin EL - (one reports 3/8" below with picture, another reports 1/2" above with no picture)
Full Race - ??
OEM - clears?

Any testimonials for clearance with the frame for the above choices?