the dunstall tank on the 400/4 is not the same as the dunstall or dresda tanks that fit the cb750 and 550 (I think that the tank pictured is actually an old dresda not a dunstall but it could be). A stock sohc tank can look good if massaged properly. I really like this tank and it is just a stock 69-76 tank with the emblem goles filled in and an old 60's honda motorcycle badge on the tank:
for reference the 1969-1974 tanks have a right side petcock, 1975 and 1976 have a left side petcock. see which will work better with your setup.
dresda still makes a universal manx style seat, same one they made back then and it looks like the one on that 400 four (because it has the cutout for the dresda over the tank mount).
anyway, omar's DTR has a cool manx fly screen that looks like it will work with a lot of bikes. see their yamaha xs650 conversion kit.