400's are they any good?

I think the 400 or any low compresion engine needs to be looked at carefuly and have some time spent on it with a mic to know exactly where you stand.

Where does that piston sit in the hole? How far down is that thing?

How many cc's in the head chamber? How far can I mill it down without creating issues with head sealing?

Once milled, how much valve lift can I get away with?

Sealing the intake a problem? You have to mill the head on the intake face, but, there is only so far you can go. There can be a problem with milling away the land where the valve cover gasket sits. If you take to much off, the valve covers will not seal.

How much is to much? (IDK.)

If the valve cover rail gets thin, you'll have to go to the intake and mill that, creating a one off intake for your combo.
Write down that number should you want another intake because you'll need to mill that intake to fit before useage.