400 with TF 240 heads - valley plate options?

You won't have any issues as long as you aren't trying to make a lot of power.
The current 440 I've built with a 3.75 stroke has Indy Heads along with the Indy valley tray with removable center section. Decent compression along with a fairly aggressive solid lift. I'm going to be somewhere in the 650hp range. That's about the limit I wanted to go because of the stock block.