Here's the result of my latest effort:
A 3/8" to 5/16" stainless adapter with proper hose beads.
This is the prototype. I still have to fine tune the process so I can make them quickly.

That is so needed. I'm in process right now of figuring out how to make that transition work. Installing your SS fuel lines, relocating fuel pump forward...yada yada.

Wanna send a prototype? I need it now. I'm used to testing things in my orthodontic practice and writing up results.......

I've always wondered about the adapter hose. Where does it do it's work. I can't find anywhere on my 73 1.7 where there seems to be a difference between one and and the other.

Second question. We hear frequent rants from a trusted source about the evils of replacing metric fuel hoses with British sizes. I assume that for hard lines that is not a problem because the bell forces the seal. Correct?

According to the parts manual it goes between the plastic supply line and filter.

The factory hoses were 9 and 7 mm. Thats about .380" and .280" dia. My SS lines are 3/8" and 5/16" or .375" and .312". 3/8" hose fits tightly enough onto the 9mm hard plastic but 5/16 hose is loose on 7mm plastic line.

Had a blonde moment there - we are talking OD not ID. 9mm is actually .354" - that would be outside diameter on the hard line. That means your imperial hardlines are slightly larger than the metric plastic tubes - the fit is going to be tighter not looser.

Substituting imperial for metric on rubber lines (inside diameter) = loose fit. Not good.
Substituting imperial for metric on hard (plastic) lines (outside diameter)= tight fit. Good.

No blonde problem here, just a brain fart. I multiplied 9 x .040" (I know .0394 is more accurate) and came up with .380"

Maybe the second prototype. This one stays here.

The blonde moment was mine for forgetting about inside diameter and outside diameter. Being a blonde (no that is not grey!) I don't multiply I just grab an online metric conversion tool . Regardless of the exact size my point is there is no concern about substituting imperial hardline for metric - it will be tighter.

ID is more important when it comes to fuel flow rates. The stainless lines also have a bigger ID (.305" and .242") than the oem plastic lines.

Great idea!!! you should throw one or two in with the SS fuel line kit! I spent about a half hour in Home Depot looking for 5/16 hose barbs and never found them! Luckily I am a machinist so I made some on the lathe.
Of the entire fuel pump relocation and SS fuel lines swap I did that stinkin adapter took the most brain power to figure out! Keep it up!

I just buy a 5/16 male hose barb and a 3/8 female hose barb. Then thread them together with some teflon tape.....

That's what I've been doing too.

The factory adapter hose is like $28.

Nice work Chris!