Why not a 460 if you have room for a big block?
I like both the 400M and the 390 and I believe they are close in HP and Torque.
The same closed chamber heads that came off the cleveland will work on the
400M. In 1961 and 62 there was a Hi Perf 390 with 10.6 to 1 that made 375HP.
Now the 400M only came out with 9 to 1 compression with 260 HP with a 4V.
A 390 with 9.5 to 1 is 265 HP so you can see that the numbers are close.
But take some after market closed chamber cleveland heads and a good carb with
after maket intake and headers and I believe the 400M would make the Hi Perf 390 look sick. If I had a choice I will allways take cubic inches though. Kurt