The following pics illustrate the differences between the Model A and B Ford stock flywheels, as well as lightened flywheels.
This is the stock Model A Ford flywheel at approximately 65 pounds. Note three machining steps in the OD.
The inside of the stock Model A flywheel.
Pictured above is the stock Model B Ford flywheel at approximately 53 pounds. Note the difference in section and the two machining steps on the OD. The Model B flywheel is smaller to fit inside the Model B flywheel housing and oil pan extension.
A stock Model A flywheel cannot be used with a stock Model B housing and oil pan. However, a stock Model B flywheel can be used on either a Model A or a Model B.
Shown above is the inside of the stock B flywheel. Note the added machined relief in the inside plane.
This is a Model A flywheel which has been cut down to 26 pounds. It still retains the stock pressure plate which Ford used on Model A, B, and V8 until 1935.
The inside of the lightened flywheel, showing substantial relief. I ran this flywheel shown above on a Model A engine for a number of years and I like the throttle response with it.
The whole point of lightening the entire mass of the rotating assembly (flywheel, disc, and pressure plate) is to decrease the mass moment of inertia to provide faster engine response (acceleration and deceleration).
Flywheel mass reduction does not have any direct correlation to the maximum power output of an engine, or the top speed of an engine. It does change the amount of energy the flywheel can store, and it does have an effect on the responsiveness (quickness of RPM change).
In regards to lightening flywheels, it does make a difference where the material is removed. Removing 15 pounds near the OD has a completely different effect than 15 pounds removed from near the centerline.
The actual mass of the flywheel is not particularly important. What matters is the rotational (mass) moment of inertia of the section (mass distribution) about the rotational axis (not always the same as the geometric centerline). The mass distribution of the pressure plate is also a part of that effect.
With a stock flywheel there are a limited number of places where material can be removed without interfering with strength, the starter ring gear mounting, pressure plate mounting, etc., so most cut-down Model A flywheels tend to be very similar in final section and arrangement.
Pictured above is a Model A/B flywheel which has been cut down by Bill Stipe to 37 pounds. It uses the V8 pressure plate which Ford introduced in 1935 and which is centrifugally flyweighted.
Stipe Machine & Specialty Motor Cams web site.
The 'weighted' V8 pressure plate has flyweights which increase pressure on the disc as the rpm is increased, but allow for lower spring (pedal) force, and easy clutch dis-engagement at low rpms and are easier to operate.
Notice the residual groove remaining in the flywheel face after the original pressure plate mounting ridge was machined off. This slight groove and the pressure plate mounting holes remaining from the original Ford machining operations are no problem, and are beyond the working diameter of the driven disc.
The flywheel face should not be further machined in an attempt to remove the groove or further "clean up" the surface. The reason to leave it alone is that any material removed from the friction contact surface moves the driven disc hub closer to the heads of the flywheel bolts.
There is already minimal (and often insufficient) clearance to the bolt heads. Any contact will cause the disc hub to hang up and not fully disengage, and will require grinding the flywheel bolt heads down to get the clearance back (not a good thing!)
Shown above is the inside of Bill's lightened flywheel for use with a V8 pressure plate.