My Open Range has some kind of off brand Chinese fixture (Pai Ying)with two 921 lamps but they face each other toward the center with a mirror reflector above the center. The spacing between sockets is only 2 3/4". It looks like fixture modification or replacement will be necessary in order to use the panel type LED's. Even the paddle type looks too large. I hate to make permanent modifications to all the fixtures but I can't see any options.
I found the rating for the 921 lamp to be 264 lumen. The 5050 LED is rated 15 lumen each (5 lumen per imbedded LED) and the 3528 is rated 5 lumen each. Reducing the light output is not an acceptable option for me. Maybe a single 48 LED 5050 would work.