Alright, I've put this off quite long enough. Lets go over the primary length design aspect.
First off, headers by ed basically says to use 'at least 36" long primary tubes'. Not every scientific IMO. But, it really kills the idea of a 'shorty' header.
Next up I gathered some information saying that the length of a primary tube should be equal to the volume of the cylinder its evacuating. The Metro engine has a 993 cc engine, so that gives us 331 ccs (20.2 cubic inches) per cylinder. With an inner diameter of .62 inches, we need a length of 67 inches to achieve that volume. Well, that definitely agrees with header's by ed theory.
Now, if we use
Old Tele man's calculation, we end up with a optimal length of 92" long.
So, what is a guy to do? Well, first off, most of those lengths are unrealistically long. We have to put the catalytic converter right after the header, and ideally we don't want to push it farther back into the exhaust. The farther back it goes, the cooler it runs and then it doesn't really do its job. So, that alone is probably going to restrict us to somewhere around 40" long. My specific case is currently drawn up at 37" because... well thats just where it is. When / if I actually get to building it, I'll probably adjust the length based on the materials I order. I believe come in 5ft lengths for the straight pipe.