You gotta see this ......
I will add one thing I haven't seen followed up on yet. Did you check the piston to valve clearance on this engine? I see this is a 340, and early 340's have pistons that are proud of the deck. I don't know the engine build specifics here, but Edelbrock sells early 340 specific heads. Are the Eddy's the one's for the early 340's? Usually that's a piston to head clearance issue, and of course again the piston to valve clearance depends on which pistons were used and the cam. But if the valve to piston clearance was marginal it might have been ok for awhile, but if you then ran it a little hotter or a little higher in RPM you could have smacked the valves with the piston, and that could have punched the rockers.
Not sure if that's the case here or not, but it is
definitely something I would check before I just changed the rockers and moved on. I had a couple of rockers that were cracked on the 318 in my '71 GT when I got it, that's exactly what happened with it. On further inspection I found a couple of slightly bent valves, a couple of bent pushrods, and a couple of dimples where you could see the valves smacked a couple pistons. On that one it was a timing chain issue, but the result would be similar with insufficient piston to valve clearance. And if the clearance was borderline it might not have happened right away, especially during break in if you were taking it easy on the engine.
Regardless of adjustable or not, you still need to measure the length.
Exactly. You have to measure for pushrod length. The deck heights on these engines are all over the place. Add aftermarket heads, a different thickness head gasket, aftermarket cam, lifters, etc and the only way to really get it right is to measure. Sure, hydraulic lifters can make up for some variance, but you can only know if you're in the right range for the hydraulic lifter pre-load if you measure. The 340 in my Duster and the 318 in my Dart have completely different length push rods and neither one use the stock length, even though the 340 uses stock heads.
The Eddy heads are based on stock geometry. There's no reason you can't use the stamped rockers on those heads. But the stamped rockers weren't designed for anything more than the stock valve springs, so unless you run a stock cam with stock valve springs (or close to it anyway) you should upgrade the rockers. They don't
need to be adjustable, but pretty much all of the stronger rockers are adjustable.