That all depends on what you are wanting to get out of your 6.

If you want the best cyl head available, Sissell's is the one to get.
He has manifolds etc, to complement most all your needs.

http://sissellautomotive.com/12%20port.htm#Accessory:32Anchor

http://sissellautomotive.com/index.htm

There are other things to consider though.
No brackets are made to bolt onto the cyl head.
No power steering, A/C, smog pump, etc, you would have to make your own.

IIRC, Turbo6 calculated Douglas' Chevy Opala 250 6 w/Sissell head was making between 1000- 1200 HP

N/A, 250 engine, Mike Kirby built a very, very mild 250 engine with his 12 port head, was in a magazine & made 320 HP.

MBHD

12 port SDS EFI