Most of this applies to late-model vehicles as well, though some of these cars-Camaros and Firebirds in particular and most trucks-have to run single systems. For these situations, a large single pipe can usually provide ample flow; 3-inch F-body after-cat systems are quite common, and 4-inch pipes are also available for hairier combinations. Ditching catalytic converters was the norm in the '70s and '80s, but it really isn't necessary anymore. Even the factory cats from the last decade usually flow so well that they only hinder performance slightly, if at all. Aftermarket cats have shown that even highly modified engines can live with smog controls without major sacrifices. Staying legal no longer equates to low performance.
One of the classic modifications of hot rodding involves installing tubular exhaust headers in place of the factory cast-iron manifolds. As most of us know, one of the most rudimentary rules of rodding states that the easier it is for the exhaust gases to get out, the more power the engine can make. So, obviously the part of the exhaust system that attaches directly to the engine is the most critical.
Back in the day, choosing headers was fairly simple. You bought a set of full-length tubes that merged with a four-into-one collector and joined the exhaust tubing with a flat flange that used a gasket. Those sorts of headers remain the mainstay of the genre, but you also have to be concerned with primary tube diameter and collector diameter. An alternative to a full-length header is a "shorty" design. Of course, the shorties were initially designed for late-model vehicles where emissions are a concern, so that the catalytic converters and other smog devices could remain intact and in their original locations. In recent years, some header manufacturers have found that a shorter header can be advantageous in a non-smog application, particularly if ground clearance is an issue. Most of these headers are considered 31/44-length, as they're longer than smog-legal shorties, but don't extend all the way to the vehicle's toe-board like a full-length design. Usually, 31/44-length headers position the collector at about a 45-degree angle to the ground, terminating in the area of the oil pan flange. Dyno-testing has shown that despite their abbreviated dimensions, 31/44-length headers can actually make more horsepower than comparable full-length tubes, but often deliver significantly less peak torque.
Which headers you choose will obviously have a lot to do with the car you're building. For late-model stuff that is still subject to smog laws and tests, certified shorty headers with an exemption number are the way to go. For other cars, the intended use is more of an issue. For example, installing a set of full-length tubes with 2-inch primaries and a 4-inch collector on a 300hp small-block for street cruising will prove to be an expensive lesson in overkill. Be realistic about your car's potential and recognize that if you go to the track twice a year, it might not be worth dragging full-competition tubes across every driveway and speed-bump you encounter in your daily travels.
Conversely, if you have a high-compression big-block with a big cam, don't cheap out on the entry-level bargain tubes if you're looking for a significant performance gain.
This is another area that has become far more complicated than it might have been 20 years ago. But the upside is that research and development of aftermarket manifold offerings has taken great strides since the early '80s, so the benefits of upgrading are better than ever.