My experiment of heating up the brass fuel tap to hammer in a central snozzle resulted in this:
...which I found to be no tighter than if I'd just hammered it in cold, so my conclusions are that brass does not expand much when heated so it would be a bit of a bugger to press fit something in it to my satisfaction.
This led to the idea of threading the inside with a standard M6 tap so I could screw some sort of threaded snozzle in it.
I have a little snozzle the right size but it's not threaded. Unfortunately I don't have a die to thread it so I'm contemplating something else.
If I could find the right sized drill bit I could just drill out the hole a bit more and hammer the snozzle in it, but I don't really like that idea as I don't trust things pressed into brass and I wouldn't want to find my fuel spewing down directly above my open bell mouth carb one day out on some lonely highway (although the resulting fireball WOULD be a pretty cool video).
So I'm back to the threading idea.
There's a lot of wall in the fuel tap so I could drill it out some more and tap it with a bigger M8 tap, which I already have, and then I could cut the head off a bolt and drill a hole through the middle of it and screw that in as a fuel hose connection.
I've got the correct adapter to fit this fuel tap into my tank, so in theory I don't have to BUY anything, just drill out and tap the brass bit and then drill a hole through a headless bolt as a hose connection.
Any reason why this shouldn't work?
The whole universe failed, I was the only thing in it that was correct.