I wouldn't recommend using the stock 2 piece retainer at that rpm.  Use aftermarket retainers and springs.  I use 427 low riser springs and 429/460 retainers.  Been over 6000 many times.  You'll need tubular pushrods with stiffer springs too.

John

I've been running stock retainers and FE 406 springs since 63. Had one failure, a broken spring. The valve is still with me. These springs probably have 80,000 miles on them but I've kept under 5500 rpm. My goal is a 6500 rpm ceiling this upgrade. However, the engine will likely never see 3000 miles a year again.  

I already added tubular push rods this summer. In going with new retainers, I was leaning toward 787 retainers, 26915 bee hive springs and under cut valves for less inertia.

Would you rule out the bee hives over a 427/429 option?  

Jerome 

The 406 springs are the same as low riser 427.  They'll go 6500 in a Y, they went there with heavier FE valves.

I wouldn't be afraid of the beehive springs.  Keep in mind you only need about 100 pounds on the seat and 250 pounds at full lift to go to 6500.

John