Andy,
I did some research here at work (I work for an aircraft manufacturer) with an engineer buddy of mine and what I found out is that 1/8" NPT (similar to BSPP?) pipe thread is 27 threads per inch, whereas 1/4 & 3/8 are both 18 tpi. I haven't had a chance yet to go look at what I have on my IDFs, but I believe I do have the fine thread (27 tpi) 1/8" NPT fuel inlets as you have indicated. I will check it out today and let you know for sure.
Mike
BTW Andy, I'll expect to see some pictures of your car and induction system....
The photo I've attached really doesn't help much, but it may give you some idea of what you need to buy.