This is not an extreme departure from yours, but I used to use Isky # 8005-A. They have a rate of 400 lb/in. I'd run them at 1.850 installed ht. which = 145 lb. I never ran a .600 net lift flat tappet. I used to use a Lunati .600 gross lift cam. That had a net lift of about .550". That gave me an open force of 365 lbs. The engine could go as high as 7600 before valve float using steel retainers. I generally used original max wedge adjustable rockers (not the garbage ones that MP sells). I never removed the inners to break in the cam. I'd think, maybe, you could go to 400 lbs. open with a cam that has been broken in. One thing worth mentioning, is that if you use 1.6 or higher rocker ratios, you'd have to back the spring down by a proportional amount to prevent wiping the cam.