If you decide not to use the #12 heads, you can probably get somewhere between 300 & 350hp, with the 6x-8 heads. Better to have some shaved off to get the CR up closer to 9:1. They are said to have aprox 100cc chambers, from the factory. Best cam for a 6x-8 head street 400 might be the Lunati Voodoo 262. But certainly not the only decent choice.
Voodoo Hydraulic Flat Tappet Cam - Pontiac V8 262/268 - Lunati Power
Here's a forged piston 6X 400 build, just for an example. Probably has 6x-4 heads tho, with less than 90cc chambers.
400 Long Block
Or, you can go with a 455 or 400 block stroker, 6x-8 heads, & easily get 400hp/500 torque.
455 Long Block
455 Short Block
No, I haven't bought an engine from Len Williams. But, I have heard good things about him. And, I do have a 6x-8 head 455 in my LOW budget bracket Bird. I put a Summit 2802 cam in it, along with Jegs lifters, to save some $$.
If you go with a 400 & the #12
heads, the Summit 2801 cam might be a good choice. But, lots of street-only 400 guys go with an 068 clone, such as the Melling SPC-7. It has barely over .400 lift, and doesn't seem to cause any of the problems that some higher lift cams & stronger valve springs cause.
If your budget will stand it, some 74cc KRE D-ports, or Edelbrock 72cc D-ports will make a good street 400. Can use Speed Pro forged flat top pistons. 10.5 CR is OK with alum heads. Don't need any porting. They flow better than iron heads, out of the box.
Kauffman Racing Equipment