There’s a bit more to consider than just swapping out your existing manifold… First, about increasing power… Yes, your current manifold is purposely designed to slow down the flow of spent gases, and an earlier (’72 - ’74) set-up will restore the quicker gas flow. However, that’s only the first restriction. The second one is the catalytic converter. The OEM cat is small and also chokes the system. The third is the diameter of the pipe from the catalyst on back. If you don’t improve the flow through the catalytic converter, you won’t get the benefit of having installed the better manifold/headers. As far as the diameter of the pipe, it has a much smaller impact. Doing headers and a cat, without touching the cat-back system, will still yield a nice improvement over the current stock set-up.
With regard to emissions, note that the increase of gases flowing unrestricted through the headers also means that the level of pollutants will be higher. Thus, this is another reason why you should replace the catalytic converter with a modern one that is not just free-flowing for performance, but also much more effective and can better handle the increase of pollutants. This should allow the car to still remain compliant, with no net increase of pollutants coming out the tail pipe. (I must note that I don’t live in CA, but all ’85 Spiders are the same in the U.S.A., and NJ also has strict emissions regulations.)
As far as the fit, there are essentially a couple of choices. You can go with the earlier ’72 - ’74 headers, but this also means that you need to change to the earlier set of down pipes. These are the front down pipes that go into the catalytic converter. The earlier pipes were never attached to a catalytic converter, but they do feed into a resonator. You should be able to fit the cat in the same place where the front resonator is found. You will also have to drill an opening for the oxygen sensor, since the earlier set-up never used one. All of this is custom work, of course.
The other choice to replace the manifold is to use a set of aftermarket headers, such as the ones sold by Centerline. These replace not just the current manifold, but also the down pipes, so it is a 4-2-1 system. The included “Y” pipe connector has the bung for the oxygen sensor and so you have the complete system that is needed right up to the point that it mates with the catalytic converter. This off-the-shelf set-up still requires some custom work, to fit to the cat, and then to fit the cat to the rest of the stock system, but it is straightforward.
In the future, when you need to replace the cat back system, you could go with one that has a larger diameter pipe, to get a little more increase in performance.
Last, I should also note that you need to be aware that these modifications, while compliant with emission levels, are still illegal to use on street cars. Of course, a visual test is only as good as the knowledge of the inspector, and none of these set-ups look out of place. It is something to consider however, as none of the above is inexpensive. On the flip side, each small power increase is noticeable, and satisfying. Finally, as an added bonus, the increase in efficiency should yield an increase of about 2-3 mpg.
Best regards,