FF has full floating hubs, weight all rides on the bearings directly and not on axle shafts so much
SF the weight rides on shafts like the 10 and 12 bolts and 44's, etc.
FF is 10.5"
SF is 9.5"
9.5" cover is oval across ways mostly
10.5" is like a big stop sign shape, flat sides.,
10.5" has hubs sticking out liek a front axle. but not that far out, and has bolt heads all around it on it, the common 76-up chrome aluminum yellow black bowtie hubcaps will fit over them so dont assume just because it has the hub caps on it thats its not an FF
9.5" has hubs just like a 12 bolt or 10 bolt, etc,
good luck