There has been some discussion about what flywheels and starters the 351M and 400 engines take. And, from the attached 1980 - 89 Ford Master Parts Catalog entries I can see why. But, Bubba's site clears that up very nicely. It says "Both 351M and 400 engines have the same 28 oz-in crankshaft imbalance, so all M-block flywheels have the same balancing counterweight, and they can be interchanged without vibration or balance problems." But, Ford muddied the water in 1985 when they issued a replacement manual transmission flywheel (E5TZ-6375-H) that is drilled for both 11" and 12" clutches - as shown on the 1989 era MPC captures below.
- Automatic transmissions: Both engines take D1AZ 6375-A flexplate, which is the same unit used in cars from 1971 forward
- Manual transmissions:
- 351M: D7TE-6375-A This is drilled for an 11" clutch, and probably used in trucks from 1977 forward given the "D7". Tim Meyer says "11" flywheels have 3/8" evenly spaced bolts."
- 400: D7TE-6375-B, which is drilled for a 12" clutch, and probably used in trucks from 1977 forward given the "D7". Tim says "12" flywheels have 5/16" bolts in 3 pairs."