1. I'd ask a moderator to combine my two threads on the
same subject.
https://www.gmt400.com/threads/full-fuel-line-replacement-what-would-you-do.46683/
2. I'd use whatever GM used, pulled directly from a salvage-yard vehicle. Cheap and efficient. If I absolutely had to buy "new", I'd be forming double-wall seamless steel tubing because it's cheap, light, durable, and easy to work with. Combine with the shortest segments of hose I could get by with, to allow for engine shake and serviceability.
Using excess hose is crazy. Hose is expensive, and if it's rubber-based, it's porous, too. PTFE/Teflon liner hose is REALLY expensive, but at least it's somewhat durable and non-porous.
For the record, it's really unlikely that folks are using "AN" fittings, as that's an obsolete Military spec of supreme quality, the replacement spec is suitable for aerospace use. Most likely, you're using the non-aerospace grade of 37 degree flare fittings which used to be called "JIC" but are now covered by an SAE spec which I'm too lazy to look up.