I was going to get some ebay Ikari sticks, then saw a mention of the interface board, and wondered if the opti-pac would run the happs, or if they were replacements for the ikari type...now I know...maybe! 
Happ has 2 rotary types, "optical" and "Ikari" type. Optical has a free(sort of) spinning action, while "Ikari" has clicking lock positions.
Optical can be used directly by OptiPac but depending what game you want to play, and if you are looking for authentic feel, it may not satisfy you.
Ikari type is the replacement for the original SNK yellow click types (Ikari, Guerrilla war, Time Soldiers, etc). Ikari type needs the extra interface before going to Ipac or other keyboard encoder, but if you want the original feel for those games, it's the most accurate reproduction of gameplay.