38 Flattie Valve stuck

[Tony Smith]

The stem heads on the original mushroom valves were quite large and spread the pressure on the face of the lifter over a large area. However, the 8BA/Chev stem head is much smaller and concentrates the pressure. The original lifters were hollow and could bear the weight of the mushroom head, but the Chev stem could deform or even punch through the original lifter.
Therefore, original valves can use either the original non-adjustable lifter or the new adjustable type, but the new 8BA/Chev style can ONLY BE USED WITH an adjustable lifter to avoid the risk of punching through a hollow lifter. Ford had a solid lifter to use originally with the 8BA engine, but even if you could find one of these now, there's no adjustment and you have to go through the tedious process of grinding/brazing each valve for clearance. Go adjustable!