We have one that is .040 over with no problems but it is very mild.

you can go .060 on a 400..helps if you run a longer rod to get some of the pressure off the cylinder walls..its best not to go over .040..but my engine is 400 is .060 over with a 4 inch stroke and a 6 inch rod in a stock block with splayed caps..been running it 7 years without failure..

.060 over isn't too bad for a 400. As long as you have it torn down, sonic test (bout 60 bucks) would be best, also consider filling it to the bottom of water pump jackets and make sure the steam holes are open on the heads.
I have 2, one at .030 over and one at .040 over. Had the one .030 sonic tested and the report showed I could go comfortably to .060.

alot of guys do fill the blocks..personally i dont..i feel like if the metal is going to expand as it heats the grout will prevent this and cause a cylinder to split..just my opinion..ive had very good luck with my block prep in the past...not to say anyone is wrong..thats just the way i run my stuff..a man should do what he feels is his best course of action