there isnt a lot of aftermarket support for the 400. Its really starting to grow. Somethings you could do would be Cleveland heads, and other cleveland parts. People more knowledgable than me will give the lowdown on exaclty what to do. None of your big online vendors are going to have much for the 400. There are smaller shops that have parts.

What are you looking to do? Complete rebuild? just a little more power, cam swap? intake? Heads?

Heads can be used from any Cleveland engine. Stock rods can be bushed for Cleveland pistons. Or a few smaller shops have pistons for the 400.

Any cam for a cleveland will work.

Edelbrock has a performer intake for the 400 which i think is only marginally better than stock. Any cleveland will work with spacers that can be bought from a few different vendors.

Im not sure of the contact info for the small vendors of the pistons or intake spacers, but somebody here will come in with more info, You have def come to the right place.