400 head has a port for the hot start octapus in the intake tack. There is a nipple pressed into a drilled hole. The boss is there on a 426 head, but it is not drilled, so if you are going to use a 426 head on a 400, you will have to either drill it and press in a nipple, or use a later model carb that has the hot start internal to the carb. If you want to use a 400 head on a 426 9as I have), you will need to remove the nipple and plug the hole (I tapped it for a 6 mm screw and plugged it with a screw wet installed with Permatex
No 2 Gasket compound), or simply cap it off with a vacuum cap (I would put the cap on with something like permatex
or RTV and then secure it with a hose clamp or zip tie to be sure it doesn't come off).
Valve seats are the same, as are the guides and seals, however the 426s use Ti valves and the 400s use SS valves...need to use the appropriate springs, keepers, etc for each.