Which airframe?

You'll have to get a fan that is pre-drilled to mount an OS77/91. The OS77 and 91 both mount the same. The best match for an OS77 is a Dynamax fan. If it were me I'd pitch the 77 and get an OS91, you'll be a lot happier. I preferred a OS91/Ramtec combination as long as the inlets were large enough.

Joe