Will a zz454 fit under the hood of a 67 SS ?
My 396 with an Edelbrock RPM manifold and 3" air cleaner just fits . Has anyone done a zz454 engine swap ? Will 2 3/4 " headers be OK ?

The ZZ454 has the exact same external dimensions as your 396.

2-3/4" headers are pretty enormous for a street 454. 1-7/8" or 2" would be preferable.

The trick to geting some engines to clear hoods seems to be to use a stock style drop base air cleaner.

Thank you . Sorry.... I have 1 3/4" headers on my 396 . Will they be OK ?

They're not ideal. Are they full length headers? What does the rest of the exhaust consist of?

They are hooker supercomp , 30" x 3" collector .
2 1/2" exhaust with Borla XL mufflers .

Considering GM underates their crate motors, I would step up to 2" headers. They don't have to be super comps.

Just completed this installation in my 67 Malibu. New zz454, th400, 12-bolt 3.73. Headers for now are 1 3/4 Hedman ceramic coated with 2.5" flowmaster exhaust. Want to move up to 2" but already had these headers. Used big block engine frame brackets and energy suspension mounts. Installed the GM oil pan that you are supposed to use in early Chevelles and Camaros. Was still a little tight at the cross member. May want to consider aftermarket oil pan. Using the factory intake that comes with it and 750 Edelbrock carb. I'm using a 3" drop base Moroso air cleaner. It fits but is hitting the hood insulation. The motor mounts are very tight so the motor doesn't move around much. Planning to switch to an Edelbrock Performer RPM manifold and Barry Grant 850cfm carb when I do the header swap. This will require an upgrade to a cowl hood I'm sure. Looking at the 2" goodmark. Hot Rod did an article on the zz454 and tested some different manifolds and carbs. Made some better numbers than what GM advertised. Really like the power and driveability. Hope this helps out. Any thing further please email me.

JMHO, doubt 2" headers will make any noticeable difference over 1.75" primaries with anything close to the stock cam. I intend to run my 502HO with the 1.75's I already have. I'm going for huge tire shredding, asphalt melting torque. Incidentally, I spoke with someone that does quite a bit with GMPP crates. Probably old news here, but the cam in the 454HO, 502HO and ZZ454 was designed for marine application running wet CI manifolds. These are very restrictive, probably on par with factory CI manifolds, maybe worse. Any size header is a huge improvement in exhaust scavenging.