I disagree - Harley engines with that stroke have been raced for 50 years. The stock Chief 80 is 4-13/16".
They break, but they run pretty well until then. The limit (other than the strength of the pieces) is rod ratio and breathing.
Since valve-to-piston contact isn't possible, valve float does very little damage unless you persist in running like that.
I wouldn't base the spring rate on that factor, especially since (with the same valve train dynamics) the spring for 6,000 RPM is 41% higher than 5,053 RPM (4,000 f/m, which may still be above where your power curve ends).
E.g. if you need 250 lbs. open pressure for 5,053 RPM you need 353 lbs. for 6,000 RPM.
