The problem with exhaust headers is not wasting your money. First, larger headers really only help at the top end, and if done right maybe only get you 3 hp max. That kind of improvement only makes any difference when racing the car down the back stretch where 3 hp will get you inching by the guy without the headers. Second, it's the bottom end where you will have trouble with all that carberation and getting the air/fuel velocity up. I've heard it said that having the smaller headers actually has a very small improvement on the bottom end in that situation. Third, and most importantly, getting them "done right" is next to impossible. The only one way to know if they are done right is to have each one put on the engine and dyno it. Otherwise, you could actually be making it worse. So, considering the price for headers, it is of questionable return for your dollar.
There are a number of suppliers out there. If you are looking to improve performance, and want a small bit of assurance that you are getting it, the only one I would buy them from is Holbay. There are a number of suppliers that make stainless steel headers which are not really designed for performance, but just replacement of worn out parts. Most of the places I spoke with when I was looking did have "performance" headers for earlier cars, but not the Series V. The Series V requires a special bend in the front tube to get around the alternator. I do not know why that would prevent a performance header. Maybe they just never designed one before they stopped buildling the Alpine. There is a place in Birmingham Alabama called Paeco that does list performance headers for the Series V. However, you have to pay in advance, the money is non-refundable, and I have heard horror stories about their product and customer service. Holbay is in England and designed the H120 head and cam. Over the years they worked on racing cars using the 1725 engine. So, I trust them to make something that works. I don't know however if they are still in business. The main guy died a year or so ago and I have not heard if they kept things going.
Certainly let us know what you do.
Jay