Ok, so I was incorrect in my assumption. The inner spring is sitting up much higher than the outer spring. This is definitely my bind issue.

I will need to look more closely at it, but I believe the heads were machined so that both the inner spring and the outer spring are at the same height. 

Ted is so right on the control of valve springs. It seems I have a never ending order thru Summit for different height keepers, retainers, and shims when setting heights and pressure of springs when reassembling cylinder heads. My sons last engine I was able to eliminate the inners and on mine going to beehive style sure changed my parts bins. Too bad nobody will know and understand why have all these boxes of left over “stuff.”.....At least my son understands why we have so many tools.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

You still did not mention if the heads had been properly machined for the inner valve springs.  That’s a major player in this scenario and especially where you have rocker arms breaking.  Retainer to seal/guide clearance is also a consideration.  It’s unusual for the oem rocker arms to break in any circumstance other than where they are in a binding situation.
 
Measuring both the closed and open valve spring pressures starting with a given installed height with and without the inner springs is required to help determine if you can remove the inner spring in a double spring set and get away with it.  Because the valve lift is relatively low on the camshaft you are using, you do have the option of being able to reduce the installed height somewhat to get the outer valve spring pressure higher.  Always keep the coil bind measurement in mind as you always want at least 0.050” more than the valve lift before coil bind becomes an issue.  Reducing the installed heights can be accomplished by using offset groove valve locks, valve spring shims, and/or reduced height valve spring retainers.  Lots of options there.

Lorena, Texas (South of Waco)

How would I determine if the inner spring needs to be used or not? I had the heads machined for newer style valve stem seals instead of the umbrella style.

You don’t give any details on what the installed spring heights are which is just one consideration.  Another consideration is that the heads were not properly machined for the double valve springs.  Both springs should be sitting flush with each other at the head.  If you are using the #6005 Isky springs, those do require some machining around the guides so that the inner spring sits flush with the outer spring.  On some applications I find that the inner springs in the 6005 and 8005 Isky spring sets are sometimes not required and for this, they are simply not used.

Lorena, Texas (South of Waco)

135# on the seat sure sounds like typical Isky. For a street engine 135 is a lot to me. Our 7000 rpm flat tappet cam dirt track engine will run close to that but my 6000 rpm 6’s stay closer to 110 or under. May not have anything to do with the breakage but over time will cause some fatigue from my experience. Good luck.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

So I set one head with the 1.43s and one with the 1.54s. Both are causing coil bind on the inside valvespring. I've emailed Isky to see if they have any suggestions. The heads have been milled previously and the block was decked a minimal amount when I rebuilt it, but I have no idea how much has actually been removed over the years.

So I had planned to remove all my rocker arms that were on this engine and replace them with the rocker arms off of my old engine. Upon further inspection, the rocker arms that were broken are the ECG, higher lift (1.54) rockers and the ones on my core engine are the lower (1.43) ratio. The specs on the camshaft are all based on a 1.5 ratio rocker arm. Would I see a big difference between the two? I'd planned to swap the rockers and reinstall everything once my new pushrods came in and then rotate the engine by hand while checking for coil bind. I wonder if that slight difference in lift would keep the coils from binding, if that was ever my issue in the first place. Just trying to avoid having to take these assemblies apart over and over again, but I may have no choice. 

Yes, they are FoMoCo rockers. Isky #6005 springs, which are 135lbs at 1.750" seat pressure and 285lb open pressure.