How much Horsepower will 5.7 sbc stock replacement fuel pump support?

Free flow is 40gph

What pump I need for 400hp sbc with rochester 750?

IMO, that is not enough. 400hp will use about 36gph at WOT and you need a lot more cushion that 4gph.
I would look for something that flows 60-80gph

Thanks Griff 👍. How about the rochester flood line, most automotive pumps dont have this inlet.

Her ya go. SBC high output fuel pump with needed pressure regulator. I'd also use a mechanical fuel pressure gauge so you can set it up easily.

https://www.cpperformance.com/p-7413...lator-kit.aspx

I'd have that on my engine if it had a pump pushrod hole. Couldn't do it so mine is electric.