Well, I've been reading your last few posts and have been wondering if you have REALLY sat down and thought out a sound game plan for how you're going to build your car and what you want it to be when it's done.
I'm NOT trying to make you feel bad, or criticize your ideas, but in my VERY humble opinion, you're going down a path that's not going to give you the results that I think you're looking for.
Trying to match a reproduction stock style fuel sending unit to a custom built fuel tank will be challenging at best, because the factory style sending units were designed to work with factory style fuel tanks.
As far as your question about the price differences on the 3/8" fuel sending units goes, it all boils down to the quality of the materials used to make the part and where they were manufactured. The cheaper units are usually made in China or a similar location, while the nicer units are made in Canada, or the USA.
Once again, I'm NOT trying to rain on your parade and I DO know how important a budget is, but trying to save money at the expense of putting parts together that weren't designed to work together is almost always a bad plan. Been working on these cars since the early 70's, so I guess that you could say that "I've been there, done that". :yes:
I'll shut up now and wish you the best of luck on your car.
Richard