Personally if I couldn't ride a qualifying 600 in 33h** I wouldn't enter PBP. But that's because I like comfortable margins and had no wish to ever be anywhere near a cutoff time, and the more so if I had time and money invested in the project.
(** And that used to be AUK's advice, by the way, back in the mists of time - actually, aim to ride a 22h 400 before considering PBP.)