Not all 2V Slants were necessarily branded as Super Sixes, but they're virtually identical other than the manifold material. My '78 ex-USAF D200 4-door had a cast-iron 2V manifold but no "Super" designation on it. Everything interchanged with Super Six Volare parts, so to me it was a Super Six. There wasn't much super about it in a 6,500lb truck, though.
The limiting factor won't be the intake manifold, it'll be the cylinder head, especially on a Slant larger than a 170. Oversized valves and porting are time and money well spent, particularly with forced induction. Intake manifold (boost) pressure is a measure of the resistance to airflow. In other words, the PSI reading is simply telling you how much air is not moving through the engine. You'll flow more with a ported/valved head and 12PSI than you will with 15PSI and a 100% stock casting. More air through the engine = more fuel burned = more power. It's no different than natural aspiration, really, except now there's pressure at the valve when it opens rather than a vacuum being generated by piston movement.
Similarly, on the exhaust side, the factory manifold will not be the major restriction--it'll be the turbo. The lower the backpressure after the turbo--the exhaust system--the greater the pressure differential across the turbine wheel. Less pressure after the turbine = quicker spool times/less lag. There is no such thing as "good" exhaust backpressure with a turbocharger.
Just like water, electricity, and human nature, any resistance is exacerbated by increased pressure. This is true on both the intake and exhaust sides.
"Street Turbocharging" by Mark Warner is an excellent resource on this subject. It might not be as entertaining as reading internet forums, but the advice is consistent and solid instead of conflicting opinions. It's well worth the price and a couple of full read-throughs to fully digest it. There's a heck of a lot more to forced induction than just hangin' a turbo on the engine and calling it a day (especially regarding ignition timing). Learning the hard way is a lot more expensive than a book and a few nights' worth of reading.
