'68 289 Valve Clicking

[dep]

I am hearing a clicking noise only when the car is accelerating--pulling a load (not when idling). The engine is a rebuilt 289 about 2 years old with only about 2,000 miles on it. I took the valve cover off thinking the valve clearance for the hydraulic lifters was too much. I tightened 4 of the rocker arms about 1/4 of a turn each, but was unable to figure out how to measure the proper distance. This tightening resulted in a lessening of the clicking noise, but I would like to do it correctly. I have a Haynes manual that looks like they are using some tool to hold the rocker arm up and get the measurement with the feeler gauge. Has anybody ever experienced something similar to this? I would like to get it sounding good before I put a new Flowmaster on it.
Thanks.