http://www.turbofast.com.au/Tfcomp.html go there, it can calculate your compression ratio for you, but you need basics like bore, stroke, deck height, gasket thickness, and Head Volume. Also you didn't mention what year that 400 was, 400s are notorious for having valve troubles around 50k or so. I know a guy that has a 400m, pulled his heads at 50k had .011 shaved off them and a valve job done, put an edelbrock intake with an 850cfm holley on top of it all and man that thing runs.