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The difference is weight and cost (as flatjack posted). The lighter the lifter the better, but in the case of low RPM flatheads it doesn't make that great a difference to justify the cost for most people. Original Ford lifter are hollow (very light weight) and the Ford tractor lifters were solid, so I guess Ford thought there was some value to the lighter lifters in the V8s or they wouldn't have spent the money on them. However almost all flathead builders claim very little gain, so there you go.