You probably have some badly worn lifters that are cupped and mushroom-shaped on the bottom. The camshaft will be worn out also. Sometimes it's necessary to pull the engine, remove the crankshaft, and drive the lifters out from the bottom side with a hammer and punch. Don't even think about changing only the lifters, and if you change tha cam and lifters, you'll end up with the same problems the 55's had 50 years ago, low oil pressure due to lots of oil loss from worn cam bearings. As soon as the 55's got a few miles on them, Dad found out that it was not a good idea to attempt an in-the-frame rebuild like he had been doing for years with the 6-cylinders. A small block V-8 with over 50,000 miles on it MUST have new cam bearings on an overhaul.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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