The problem is in the heads. 400 heads were junk and a swap to 351C heads wasn't much better. I too had an interest in this motor at one time and read extensively on it. The 351C 2V heads are an open chamber type, precluding any chance to design squish into the motor, so that you can't build any reasonable compression ratio and still run on pump gas. The 4V heads are closed chamber, but have a dogleg in them that screws up any chance of good flow. It seems that the serious builders have given up on them. There was supposedly an Australian 4V closed chamber head that worked well, but of course availability is a problem. The way I see it is that the weight of the reciprocating assembly is another area of concern if you plan on buzzing the motor.

I think most guys, if they're gonna go with a large, heavy motor like the 400, will go ahead and use a 429 or 460 where they can get aftermarket go-parts. Of course, there are always the guys who have built these motors and swear by them, much in the same manner as Scooter swears by his 283's .

I haven't checked lately for the availability of aftermarket aluminum 351C 4V heads, but if you're determined to go after it, that would seem to me to be the way to do it.

A friend of mine spent untold gobs of money on a 400 several years back, then decided to abandon the project. He found that he couldn't GIVE it away.