I'd go with the 351, but you don't need a high rise intake for the set up your talkiin about, i'd recommend a performer rpm airgap intake, it will make alot more midrange power, cause with the high rise you will have zero bottom end, if you plan on going carb'd youll have to switch out the fuel pump pickup in the tank, get one for a carb'd 84', and it'd be best if you run a mechanical fuel pump. I got a 351w in my 92' and its unbelievable how much of a power difference there is, like they say, there's no replacement for displacement!, you should have right at 385 horsepower with a setup like yours. good luck! and the gt40 heads will work fine, but they are going to hold you back over 5500rpm cause they don't breathe!, another reason a high rise intake wouldn't be the way to go.