There are a lot of variables to consider besides just the diameter of the clutch disc. In any case you know it is going to be a 26 spline and not the earlier 10 spline used with pre 1971 Muncies.
The friction material will determine how well it holds, and how temperamental the bite is. The springs in the hub will aid shifting and preserving drive line parts, though some race only clutches do not have any springs in the hub. Pucks or a full surface clutch also affects how the friction disc behaves and holds.
The material used in the flywheel determines how fast the motor will rev. On the street or on a road track an aluminum fly wheel aides in allowing the motor to quickly rev back up again (as does using light weight reciprocating parts inside your engine). Along the same line keeping the clutch diameter small aids in faster revs, another good point for multiple disc clutches that are only seven inches in diameter. A heavy steel wheel can store kinetic energy to blast you off the line if your tires will hold.
That leaves the choice of Borg and Beck or a diaphragm pressure plate. The Borg and Beck usually uses nine coil springs that are pretty stiff, but can be any stiffness you want (they are similar to valve springs and can be custom made to any pressure) if you need too much spring pressure to hold a lot of torque weights can be added to the arms to further add clamping force but this is unusual. You generally find weights used with a diaphragm Pressure plate. The problem with weights is that as you rev the motor to gain more power shifting requires greater effort to disengage the clutch.
In the bad old days when I was much younger we made crash boxes out of our Muncies by grinding off every other, or every two synchro lock teeth on the gear to slam it into engagement. This required a deft touch gained by practice, which as you can well imagine a few four speed boxes where ruined in the learning curve. But Muncies where much more common back then.
I favor McLeod in clutches, but I have never had a bad one from any of the name brands. I suggest that you contact their sales engineers and have pertinent data ready such as engine torque, peak horse power and peak torque RPM, car weight rear gear ratio, tire size, and suspension mods. They can make an informed recommendation that will fit your car, driving style and more importantly your budget.
Larger Dave
