[redmodelt]

"In my spare parts collection are nearly 3 full sets of solid tappets of varying lengths. I might be able to swap out tappets and get nominal .015 clearance at every tappet location."
Why would you go through the work to replace a few lifters, at that point you should just replace with adjustable type. It would require tearing down the engine to the level of a short block to get the cam and lifters out. With solid lifters it is the valve stem that is ground to adjust the gap. Did you take the plugs out when you checked the gap? But before you do all the work I would do as rocket1 said, check compression and leak down.