Hot Rod did a build up on a 400 a few years back that you might want to look at, here is a link http://www.geocities.com/styleline58/400.html The 351m-400 heads made after 1974 are almost identical to the Cleveland 2bbl heads except for a more restrictive exhaust port so if yours are in good shape you might consider keeping them.I got my Ferrea single groove valves from AD Performance for a pretty good price or there is a guy on ebay who sells them, but remember that single groove and multiple groove locks also use a different retainer so you have to change that as well.Another option is 351M motors made after about 1979 have single groove valves from the factory.