Economical replacement rocker arm set?

is the engine running rough? you assumed that old cam and new cam are ground on the same base circle. That is possible but not likely.
Is the ticking fairly constant? If you are really lucky, the spark is just jumping from spark plug boot to ground. Watch it run at night, and you will see the spark jumping. I admit that's a long shot but I'm a glass half full kind of guy. your next best scenario is you need to get more lifter preload. Or you have a bad lifter or two. Perhaps they can be removed and taken apart and cleaned up. Less rosy scenario is you have a cam lobe going away. Let's hope that isn't the case.