Why headers? Before anyone runs out and buys a steel pretzel, the answer to this question must be understood. Let me give you a simple analogy. Imagine a  train highballing down a very narrow tunnel. What happens to the air in front of the train? Well, it can't flow efficiently around the train, so it gets pushed ahead. If you were to put a manometer on the nose of the choo-choo you would discover high pressure. Well, if the train pushes all  that air in front of it, what sort of condition exists behind the train? Low pressure must follow the train. A partial vacuum, in ordinary words. If you stand at the near end of the tunnel, behind the train, you will feel  air being drawn in to the tunnel. If you stand at the far end of the tunnel, in front of the train (not advised), you would feel air being  pushed out of the tunnel ahead of the train.
 
What does this have to do with Morgan engines, you ask. When an exhaust valve opens, a hot, energetic blast of gas is released into the exhaust manifold. For as long as it retains this energy, it works just like the freight train....gases are pushed out ahead of it, and a partial vacuum follows it. Some would have you think that headers are all about reducing back pressure, reducing the work needed to push the gas out the  tailpipe. I don't think so as a simple iron manifold with suitable exhaust piping can flow as well as most headers. The purpose of a header is to make use of the partial vacuum that follows the exhaust pulse to draw exhaust out of the system.

Return for a moment to the exhaust pulse. It's expelled from the port, and begins to travel down the header runner. Behind it, there's a vacuum, or more correctly, an area of low pressure. The pulse travels down the tube at some specific rate of speed, determined by the diameter of the pipe, the displacement of the engine, the temperature (very important) of the exhaust  gas, RPM, cam timing, induction flow, etc. Now, at some point the exhaust  pulse will pass through the collector. If all the factors mentioned work
out just exactly right, the low pressure condition which is following the  pulse will arrive in the collector just as the exhaust valve opens on the next cylinder in that bank. So when that exhaust valve opens, it will open on....a partial vacuum, and the exhaust gas will be drawn out, or scavenged, from the cylinder.  Therefore, tuning the length and diameter of the runner is what makes the power. Since these factors determine the time it will take for  the low pressure condition to form in the collector, a given length and
diameter of pipe will create the desired effect only in a narrow range of  load and rpm.
 
Will headers actually help your Morgan? That depends on the year and configuration you have and just what type of driving you wish to optimize. If you followed the above discussion, you will realize that the effectiveness of a given header will depend on many factors, including all of the little choices that were made in designing  the engine: head flow, induction capacity, cylinder size, etc. It's also going to be very dependent on RPM. In fact, a given header system will give effective improvement in only a very narrow RPM range. What RPM range is that, you ask....well, it depends. Big, short tubes work better for high RPM, skinny long tubes for midrange. You design the header for the car, the engine, the driver. A bad design can actually REDUCE performance in the range you want to help.

So what makes a header system good or bad? As discussed, the effective performance of the car will be determined primarily by the diameter and length of the tubing, so it's good to understand what sort of driving the designer was trying to optimize. Different designs will be "on" in different situations, some at WOT, some at part throttle, some at lower speed. Bolting racing headers on your car? The I hope you are going racing....

Another good thing to look for is insulation....you want to keep the heat  IN the exhaust system....the more energy in the exhaust pulse, the better. Insulation can best be achieved by coating with a modern ceramic which allows for fast gas flow, cooler temperatures and a pretty look that is impervious to the elements and major discoloration while keep the strength of mild steel. You can also use wrapping in a fibreglass, ceramic, or mineral blanket..but his will inevitably encourage rust..

Which Morgan header is best?  By now, you should realize that this is not an easy answer. There are
few things that bear mentioning. Headers are severely compromised by space availability.  If your exhaust must make a sharp right angle turn just beyond the port this bend limits the effectiveness of anything you try.

Headers are not about reducing back pressure, they are about scavenging. No header will produce even results across the whole rev and load range. Understand what the system was designed to optimize before you buy. Don't expect a miracle...and results may vary. Avoid getting involved in over complications followed by balance pipes and the like. One thing for sure is that headers will release more power than the extremely restrictive cast Rover manifold.