Freshen 318 magnum, install 230- 235 or so duration camshaft of no more than around .515 lift add appropriate valve springs to head. No need for exotic connecting rods the factory proved stock magnum rods would handle 420hp. The magnum have hypertectic pistons from the factory. These items have been proven for 400hp from the factory and the mopar street truck book backs this up.
Ya see the small block mopar 318 magnum was and is more performance oriented right out of the factory than any other previous 318. No need for fancy balancing and Xtra super rods and forged pistons.
So 318 magnum is capable of producing 400hp without alot of exotic parts. Whether or not you want to push it to 400hp is your decision and opinion. I will just state the facts