390 HP is not any where near 475 HP, with a stock stroke motor. Stroking it and raising the compression may add another 40 to 60 HP, depending on the cam and peak HP RPM. Me thinks those motor where rated at 290 HP, not 390 HP
On your deal adding a supercharger with water and methanol injection may help you blow the stock bottom end out of it
The poly heads, no matter which brand that Mopar installed them on, where designed for low power and low production costs, not making power above 4000 RPM
I would like to see the manual you got that 390 HP numbers from
The 1957 and 1958 392 dual quad solid lifter 300 letter Hemi motors made less than 380 HP
How and why would Mopar make the 318 motor your talking about have more power in a lot cheaper car?
The 1958 Dodge D500 cars made for NASCAR racing had 354 C.I. Chrysler dual quad Hemi motors in them, check the HP rating on them