Originally Posted by Mr. P-Body
4.10s were never a Pontiac "factory" gear. T/As of that era with the 4-speed were usually 3.23s or 3.42s. The latter is the best for overall "street performance". Good accelleration and reasonable cruise RPM.
6Xs can be made to "work" on a 400. Milling about .040" off the -4s will yield right at 9:1. Get them "down" to about 88 CCs, 90 at the most (-8s are not suited to this, better left to the larger displacements). The exhaust valve should be upgraded to the 1.77" version. Porting should be done by someone that knows both porting AND Pontiacs. What makes a Chevy "turn on" may "kill" the power in the Pontiac. Jim Hand's "How to Build Max-performance Pontiac V8s" by SA Designs has VERY good porting detail.
With the manual trans, I would more recommend the XE274H over the 268. Still plenty of low-end and adequate vacuum, it will rev well past 5,500, where the 286 will be "all done" before that. If you REALLY wanna get silly, and make a 406 Pontiac a seathing monster, consider the XS series. We use SX274S in many 400s, along with the 6.8" Eagle rod. They routinely rev beyond 6,500 making power. NOTE: If someone tells you you "can't" rev a Pontiac, send them back to the '70s... Remember, when a Pontiac is running, it's making torque. It's very difficult to "hurt" the low-end. Add that to the power peak above 6,500 and you have a very strong street engine with a BROAD power band. I highly advise AGAINST the "K-Kit'. It contains the "wrong" timing set AND valve stem seals. Oh, they "fit", but they're not the ones we would use if building an engine for a customer. The timing set is the cheaper "2112", where the 3112 is called for ("true" roller, not a glorified bicycle chain). The Teflon seals are okay for a race engine, but we use Viton in street engines today. Also, there are other spring providers that have good offerings for the "odd" heights the Pontiac calls for. Reports on the Comp 995 have been spotty.
IMO, the best intake for the 400 is the RPM. This presents a hood issue, as you said. Larry Navarro of WFO Enterprises makes a "drop base" air cleaner housing for this purpose. It allows the use of the "shaker" AND a good intake. I would also recommend the AED 750HO or HOM carb. An MSD "Pro Billet" will cap it off. This combo is well over 400 HP and around 475-500 lbs. of torque.
There will be plenty other "opinions" on this. You will be advised to forget the smaller engine and "stroke" it to 461 or bigger. Good advice, but not MANDETORY. 400 is the engine that made GTO famous, and it wasn't famous for getting beat... A 400 is no punk when properly prepared. Vinny Meyeda goes 6.90s over 200 with a stock-block/crank 406 in his Probe F/C.