In that case (here comes the hate) I would take the Chevy.
The deck on the 400 is so tall and the stroke so short that the R/S ratio is on the high side anyway and the piston would be horribly top heavy.
You start getting to 8k plus with a piston like that and your ring seal (under load) will be compromised too much.
Even if you coated the skirts and you were running with a clearance of .0015 (or less) it still would be far too compromised to be competitive.
If you could get a 400 block with say a 9.200 deck that thing would smoke some ***.