From where I sit........ which is from a position of having not tested any of your stuff...... it certainly appears as though that spring, in that application, with that cam/RR ratio........ isn’t a match made in heaven.
I doubt fiddling with the installed height is going to fix it(but never say never).
I’ve tested many SBC’s with similar lift cams, using stud mounted rockers in Dirt modifieds that would go to 8K with less spring than you have........ and many of them used fairly aggressive cams.
With what you’ve got into springs/retainers/locks, etc....... it might be less $$$ to use a different cam profile and keep the springs.
If I were to look at different springs, I’d be tempted to try something with a damper..... or perhaps if you have enough available installed height, something like the new conical springs.
The only “SR” lobes I see in the lobe master catalog are designated “street roller” profiles(nothing with that lift though).
Perhaps the high rpm stability wasn’t a primary design parameter for that series of lobes....... and they just aren’t all that happy at high rpm.
short duration 304(266@50) and 308(270@50)
Do you know the checking height for the 304/308?
If it’s .020”......... that’s not an aggressive lobe at all...... at least the intensity near the seat doesn’t look to be.
Even an old Comp hi-tech.420 would be 302/306 @.020 for 266/270@.050........ and those lobes are pretty smooth.
Do you know, or did you check the duration @.200?
