What would you do?

The shop "hates iron lifters"? What has he used for the past say, 40 years? He's missing something IMO. A few "ifs"... If it's down on power (and you say it feels it), and if it keeps collapsing one or more lifters, and if it gets noisey on a regular basis, I'd say he's overlooking something. Not saying "they suck" or anything, just saying I get tunnel vision when I encounter some issues, and sometimes one needs to look outside what you swear was the problem. Did they ever measure the lifter bores? Many places dont bother, unless the lifters dont rotate, or a new lifter sticks. But too big is just as bad. It can cause that one lifter to not get sufficient pressure to keep things pumped up. And swapping it to another bore cant be done because of the wear in patterns. They used to offer oversize lifters..I havent looked lately to see if that is still true. Also, you have factory heads. What did the rocker pattern on the valve tip look like? Rocker geometry and lifter preload can also cause problems. You should already have the preload figures for each lifter written down if it was noisey once previously. IMO, replacing lifters twice is fixing a symptom, not the problem. And Schubecks are cool, but not needed here. If your builder is just looking to get you out of his face, I think you're into a long head butting session.