It isn't advisable to simply swap Quadajets around. I strongly feel it's when hobbyists try to install, for example, a Q-jet originally calibrated for a standard-performance 301ci engine onto a high-winding 474 stroker without proper modification that it lends to its negative reputation. We've had great success using any Quadrajet once properly modified for the combination, and it sounds as if you took the right approach, properly modifying yours prior to installation.
The black plastic cup you're asking about is one of those pieces that can be removed without worry. Its purpose is to fill the void that was left behind when the small altitude compensator known as an aneroid-metering rod was removed from the M4MV casting in the mid-'70s. I'm unaware of any negative issues resulting from its absence, and it actually increases float bowl capacity slightly, which may be enough to ward off fuel starvation in otherwise marginal conditions.
It sounds as if you have the carburetor operating as intended. If it's calibrated correctly, you'll find out just how versatile a Quadrajet can be. It can offer a smooth feel that mimics EFI under normal driving conditions yet provide you with the infamous "four-barrel moan" that hobbyists love.
Stuff A '76 455 In A '70 GTO
I have been a loyal subscriber to HPP since the early '80s. My '70 GTO convertible came with a 455 and has since been replaced with a 400. I have my hands on a 455 that came out of a '76 Grand Prix or Catalina. Will this engine work in my Goat (bellhousing and motor mounts)? Can I put on older heads, intake, and exhaust manifolds from a '70 series? Thanks!
Paul Spotts responds:
I've also been receiving HPP since the beginning days of Thunder Am. The answer to your question is yes, as all 455s are drilled and tapped for all the engine mount combinations. In other words, the 455 accepts the early two-bolt and later three-bolt mounts.
You need to research which engine mounts are on the 400 block right now. The '70 455 used the three-mount system, which was new for '70. The '70 400 used the early two-bolt system. Not only are the mounts different, the frame brackets are different as well, and do not interchange with each other. You can still use the two-bolt mounts on the 455, if the 400 has them installed. In other words, you can use the system in the car, as long as everything is lined up correctly. Pontiac came out with a stronger mount system with the new-for-'70 engines that had more torque.
Also keep in mind, you do not need a 455 block to get more cubic inches anymore, as the stroker kits allow a 400 block displacement to increase to 495 ci!
The basic Pontiac block from the mid-'60s has the same parameters to allow interchanging of the timing covers, heads, and so on. With your car, seriously consider either the correct WA (manual trans) or YA (auto trans) coded '70 block or a service replacement block. These are hard to find and can be expensive. Your GTO was originally a four-bolt main 455 with #64 casting heads and the only high-compression 455 Pontiac produced. A '70 GTO convertible with the optional 455 engine is quite collectible these days, and most owners are keeping them as original as possible.
Another suggestion is to use your '76 block and use the Felpro head gaskets as a template, then machine or grind the block chamfers. This allows the heads to flow more as the '76 block may not have those. If the block is bored 0.060, it is not as critical, but will help. You can look at a '68-'70 350 HP 400 block to see what the chamfers look like.