Ok all makes sense now, the picture of the brass fittings helps (I couldn't picture what you were planning in the context of those spring-clip fittings). So you're planning on cutting the vinyl lines (inside the rubber outer coating) and then using these 90 degree elbows close to the fuel rail.
Personally I'd go a bit of a different route, for a couple of reasons. Those lines are still going to be a bit too-close-for-comfort to the header pipe. You could lessen the risk with some fireproof sleeve and wrapping the header, but there's just not enough length to them to really get them out of the way.
The 2nd bigger reason is that those old vinyl lines may already be brittle - folks report them cracking and causing engine fires. I didn't have that problem thankfully, but as my turbo headers (more of a regular kit configuration that puts the turbo where the air-box was) ran underneath the lines and they get pretty darned hot, I wasn't comfortable with vinyl lines there at all. They soften with heat (that's how you get those barbed fittings into them typically with a little boiling water) and can completely melt. Put that together with ~40psi in the old, brittle lines that have been flexed as they are now, and it's an accident waiting to happen.
I'd personally go with this as a starting point:
https://lmr.com/item/KP-5070/1986-98-Mustang-6An-And-8An-Fuel-Rail-Adaptor-Kit
They sell braided line kits but those are usually just slightly angled. With these you can configure your lines and route them however you want.
Use some 6AN / 8AN 90-degree elbows and a couple of lengths of braided line, and some adapters on the hard-lines, and you're in business and completely safe - largely bolt-on. This will cost you more (I'd guess somewhere between $100-$200 or so) but would give you a more durable and much safer solution to your problem. I realize that's not in everyone's budget, but I try not to mess around with safety when it comes to gasoline near very hot stuff. I'd hate to see you get hurt or your car get destroyed when I had the opportunity to say something.
Here's a really good thread about the rest of the plumbing back to the hard-lines:
https://www.stangnet.com/mustang-forums/threads/1995-gt-rubber-fuel-lines.877769/
I still haven't answered your question about the vinyl fuel-line ID's, I honestly don't know as my original lines went out with the trash a few years ago. I imagine cutting your lines where you'd intend to install the barbed fittings and measuring would be the safest bet (unless someone else chimes in). Underneath the rubber sleeving the lines might also be marked with their size. But I hope you reconsider.
