My information says BOTH; the 56-57 Power Pack versions got the B and B, everything else received the diaphragm. The 58 on 348's also got the B and B. My 58 3/4 ton truck with a 55, 4 barrel 265 (?), had a B and B (the engine was supposed to be a new takeout from a wrecked 55 many years ago). Personally, I prefer the B and B. I think the old stories about the diaphragm clutch pedal "sticking to the floor" is probably the reason I had that opinion. My OT, 72 Nova SS had a diaphragm clutch, and I never had any problems with it "sticking to the floor". Can't say much for the Saginaw 4-speed it came with; it was replaced with a BW T-10 as soon as I found one. I don't know how much of an "old hot rodders wife" story, really led to the diaphragm clutch story, but I'm sure there are similar tales about the B and B. I see you're from Washington State; where abouts'? I am Butch/56sedandelivery.
If you adjusted that Diaphram to where it releases too far it can over center at high RPM and not reapply. I think sometimes the engine torque twisting and binding the clutch linkage was part of the problem.
You will also find two different style diaphragm pressure plates and two different length throw out bearings. You can use either style, but be sure to use the correct mating pieces.
You can look up your application..... http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0158.htm
The Jalopy Journal
