This is a topic I've been considering for a long time. Basically it seems there are a few approaches possible. I'll post MY OPINIONS (let me repeat...MY OPINIONS) on them:
1) A cast-iron manifold with dual-outlets; either the 1974 US manifold or one of the various Euro models. Some good points to this are: better flow than the stock single-outlet cast-iron manifold, good sound control, it's durable. Some cons are: it's very heavy compared to a tube header, it does not flow quite as well as a properly designed tube-type header, they seem to be getting more difficult (and expensive) to find (particularly here in the US).
2) A tube-type header designed for 'ultimate high-performance' / track use. This is a very generalized statement but these usually have a larger diameter / shorter runner, 4 into 1 design. They tend to offer an increase in power focused on higher RPM's...if you will, more horsepower than torque. If made from stainless steel and well built they can be rather durable. Otherwise the pros and cons are pretty much the opposite as those for the cast-iron manifold.
3) A tube-type header designed for 'drive-ability' / street use. Again a generalized statement, these usually have a smaller diameter / longer runner, 4 into 2 into 1 design. They tend to offer an increase in power that is focused in lower / mid-range RPM's...more torque than horsepower. All the same pro/con comments as number "2" above. Unfortunately I do not know of any such header available today. The old "Ansa" system was a perfect example of this design. Although the Ansa was not made from stainless steel and the muffler portion tended to rust out, the design was ideal for a light-weight street-use 'header' exhaust system. I'd really like to see someone build a copy of this using stainless steel and good build quality.
4) Various tube-type headers that are not well defined in terms of application, performance gains, build quality, etc. For some reason these tend to be very over priced. They also tend to be less accommodating for such things as air conditioning, multiple carburetors, clearance issues.
Notice I am not making recommendations / claims / implications / or reference to any seller or specific product (other than the NLA 'Ansa'). I'm only offering a little summery of MY OPINIONS (have I made that part clear yet?).
