The 516s are small valve boat anchors, They are not a decent head unless they have alot of work done to them....The 452s are a good head and if you use the comp XE cams they are designed to run with low compression motors... If you do your research has in mopar performance/direct connection race books it tells you right in to the book to NOT use closed chamber heads on a stock 400 as it hurts performance I need to look it back up in the book but it has something to do with the quench, etc... compression is important but trying to gain a 1/2 a point or so of compression by using poor heads is like farting in a windstorm, Your better off using your 452s with a smaller XE comp cams a factory highstall convertor with the 391s and it will run good.... If anyting do what was said above and save a few bucks and have the 452 heads shaved to boost the compression the biggest thing i can say is dont over cam it, stay at a 268 or even a 262 comp i wouldnt go much bigger, Those cams with a good dual plane intake will make lots of torque...