welcome!
fuel systems....
fore warning - i left the stock flex pump in place didnt touch it, sumped the tank and threw in a magnafuel 750 (close to 1k hp itself) as a secondary pump. i did this so that i could still use remote start, use the stock pump cruising around and not have to hear the big boy all day, if one dies the other can just be turned on to get home, etc. and i also over kill everything
stock flex pump was good for about 400-450hp on e85, 600ish on 93oct i started having long term issues with running stock injectors over 90% duty cycle. best guess i have for stock pump is about 380lph
i believe the stock fpcm has no problem working with a single walbro 450, but iirc they'll do 550-600hp before theyre out on e85. i think at 8psi you'll be very near that area but its always good to have some buffer.
theres the "hellcat" pump out now, walbro 525lph, havent heard of anyone running one on a fpcm. they draw a lot more amps over the 450 and dont flow that much more.
I would AT MINIMUM do a single 450, and just plan on pretty much max'd out. if you want to come out of the gate over prepared a pair of 450s in the tank is a stout fuel pump set up.... likewise a stock flex pump and a 450 should feed over 800hp easily. i think the stock in tank basket will hold two 450s, run the second one off a hobbs switch (pressure switch)
injectors....850cc to me are right on the edge again, which sucks cause after that they get expensive. you have an inverse relationship for pump and injector. injectors flow more at higher psi but pump flows less at higher psi. 850cc will flow 700hp@ 70psi but a single 450 isnt going to get you there \/ thats walbros pump curve for a 450lph