Well a junkyard jewel 360 is going to require going into the motor to get 400 HP. The crate 360/380 HP motors have higher compression pistons and a hotter cam. So basically you are talking about a rebuild so you are looking several thousand dollars.
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Im sorry my man , but my best friend is an v8 engine builder since the last 21 years , and he s a mopar la engine specialist , and the regular 5.9 magnum in the trucks for example is exactly the same compression and heads than the 5.9 380 -390 hp mopar crate engine ,, so to make 400 or with a 5.9 magnum is really easy , and its the best street strip engine in the small block familly , fiability , low end torque , good hp and its verry easy to modify as so it is so simple ..fuel , air and spark .. thats it
a lot of people dont even know what is a 360 5.9 mopar crate engine ( its a underdog ) or what it can achieve , it is a verry respected engine fo those who runs at the dragstrips ..
and even a 318 magnum , can be verry dangerous if its well done , put some aluminium edelbrock heads , port them ,better valvetrain , and a good set up of cam , carburator , and intake on a 360 magnum and a modded new hemi is just as equal or better ,, theres nothing special about the new hemi , at least you have technology , fuel economy , but still your problems ar not gone ... more you put money , and more you got result ,, thats it .. if you have money.. do it ,, but it changes nothing if you want to save on gas , with all the money you gonna pay to do the swap !