If your only restrictions are 377ci and flattop pistons,you can get way more than 650hp.If there are other restrictions that are not mentioned such as must be steel heads,or must be flat tappet cam or no porting allowed then it gets harder to do.
I get 690hp out of my 360ci steelhead engines.They all have RHS raised runner ported heads,titanium valves,roller cams and dome pistons so 650 is easy with flattops.It just depends on how good everything else is.
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