The clutch diameter all depends on what flywheel/bellhousing you intend to use. If you are using a 3 speed bell/flywheel, then they came with a 10". There are some here that found a 10.4" or 10.5" that bolts to a 10" flywheel FWIW. The 351s (or Boss 302) are the engines that came with the 164 tooth flywheel and big bell. Those use 11" clutches. Those are what I tend to use. Some 300 I6 and 302 trucks use a big bell and 11", but I steer away from those because I read somewhere that the depth is a little different on them and they don't translate to toploader well. They are supposed to damage the input shaft, similar to trying to use an FE toploader on an SBF. Looks the same, but the one little difference is enough to ruin the tranny. Long style clutch is what they are designed to use. If you go diaphram, you have to get one that is specially made for the conversion from long to diaphram. That is what the Centerforce does. The street types convert to diaphram for less pedal pressure. The dual friction converts AND has large counterweights for extra clamp, as well as dual friction materials. Edit: You can also convert to diaphram if you buy an aftermarket flywheel with multiple bolt patterns drilled in it.
