Rereading your post I see the engine lived a long life hitting hi rpms, so to be clear valvesprings do break. The original 26lb springs were very durable (White stripe) then came a batch that had a bunch of guys reporting early breakage. There were red stripe 37 LB springs and a lot of folks also had springs break and drop a valve. Now folks are using the outer of the dual green stripe springs that depending on install height give mid 30LB spring pressures and seem to hold up well. You may have not had anything wrong but the spring fatigued and broke. Floating the valves a lot will break parts also.
I did try going from the 8 tooth clutch to 9 tooth today to try and keep the rpm’s down longer, haven’t had the chance to really wind it out to see a max but did see 6450. I am unsure if I did float the valves or not, I can’t tell??
Clutch setup is the juggernaut with 6 inch driven to a 53 tooth rear on 19” tires. I’ll look into those dual springs too, but on my head, other than ss valves and split retainers and being milled .060 it’s stock including rockers for now. I’m scared to put the 1.3’s back on for fear of breaking a spring.
guess I need to figure how to tell my valves are floating or not and order some spare springs