I agree with Harold (though his last post said it backwards...) The current setup, and proposed setup, are both a bit of a mismatch. Even though Edelbrock 1406 provides bolt holes to fit on Pontiac Spread-bore manifolds, it is not an ideal airflow. The 1406 is a generic carb, designed primarily as a square bore, that in my opinion should not be run on a spread bore Pontiac intake.

They fit. It runs. But performance will not be optimal.

I feel your money would be better spent keeping your 68 manifold and replacing the carb with a spread-bore carb. If you don't want a Quadrajet for some reason you can buy an aftermarket spread-bore carb...Holley 4175 for example. I believe there are others but I haven't researched it much since I'm a Q-jet guy. I believe you are more likely to notice a slight performance gain with a different carb, rather than just putting another mis-matched intake of a different material & brand name.

That said, I think you may notice no improvements regardless of what you do with carb/intake. I'm not real strong on this type of history, but I believe that a 1974 400 with stock rebuild is simply a very low powered engine. I believe most '74s were rated 175 hp, some were 200, and I think the best was the T/A motor which was 225. It was the first year of the full blown smog era that killed the muscle car. I think they were all low compression, small valves, etc. So you may not notice any change at all with any intake/carb changes because they simply don't move much air. If you are actually looking for a performance increase, you really need different heads. Your current engine, if rebuilt to stock specs as you say, is nowhere near capable of flowing enough air to get the power potential out of ANY 4 bbl carb or intake manifold.

Now the good news is that the 1974 Pontiac 400 block (assuming it's a 481988) is that it is a fantastic block for a high performance rebuild. Many consider it the best of all Pontiac blocks...one of the strongest blocks and favored by builders over the newer ones. If course it'll take substantial $$$$ to rebuild it properly to make it run strong. But even the least expensive of "performance" rebuilds could very easily double the power out put of that engine...