With a walbro 400 and ID 2000 will I need a fpr to make 600 on e85

 

You still need the fuel pressure regulator to help control the amount of fuel your going to use.

 

I think he meant stock vs an adjustable aftermarket one

 

With the stock regulator your base pressure will likely be higher than you want. The regulator will become a restriction due to the amount of fuel your pump flows. Your pressure will likely not be able to go lower than 55psi or so.