Well, I've got the same symptoms with my fresh 455 and have had to pull the motor each winter for the last 3 years to fix my leaky rear main. My dipstick blows out under full throttle too. This time I decided to send it to antoher machine shop for a tear down inspection and rebuild because I think my rings didn't seat thereby causing too much crank case pressure. I have a breather in each of my valve covers and a PCV valve in my valley pan which is then attached to my carb spacer plate. I should know by early next week if it's what I suspect. I'll keep you informed if you like.
I agree with Grusome that your oil pressure is good.