The automatic advance mechanism is clever. However it is not really needed. The amount of advance needed depends on a number of factors including whether the engine has the original head or a higher compression head. The timing is normally adjusted by the operator to suit the driving conditions. The head is designed to provide turbulence which increases with engine speed. The turbulence makes the flame burn quicker so as the engine speed is increased about the same advance is needed. The automatic advance mechanism has a ramp which increases the advance with engine speed. This is not needed and can lead to poor performance and other issues. Plus, the maximum advance may not suit your engine or the driving conditions.
If you have the engine out of the car and on an engine stand, it is really easy to change to the adjustable lifters. The two wrench ones are preferred. Depending on the condition of the cam and how it was ground, the clearance should be anywhere from 0012 inch to more. You can get a new cam from Burtz which has good performance and is accurately ground. Changing the lifters with the engine in the car can be done but is more complicated.