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400 crank rods put a mild cam in it port don't polish. Edelbrock dual plane intake 650cfm Edelbrock. Check out tim Meyers site he's awsone with those engines.
I will add to what Mr Lopes said. And you will need to replace the timing chain with a new one (straight up) which will get rid of the 4 deg. retard on the timing. Might also have to replace some of the pushrods since with the new cam, some will come up short and the non adjustable valve train wont allow any adjustments. I got around that by installing Crane Cams screw in stud kit and a set of big block Chevy roller tip rockers that worked fine but in retrospect I should have ponied up a few more bucks for a set of Ford roller tip rockers that have the 1.73 ratio versus 1.70 on the Chebbie rockers. I ran a Carter 625 on top of an Edelbrock intake blowing the exhaust gases out thru a pair of headers and 2 1/4 glass pack mufflers. (I love this part!) I use to get all sorts of questions about my 390. LOL But my fav story is about the night I came across this guy in a late 70's Chev 4x4 bragging about his LS6
He got spanked.....