0.045" is the quotes gap for the older Fox Mustangs. 79-84 IIRC. I've seen a lot of sources carry that gap up to 1993 despite the gap on the later cars being spec'ed at 0.054"
It's probably just me but I never had any luck with Autolites and 5,0 motors . A while back I had an 82 5.0 that I threw autolites in. Ran so crappy, 2 days later put in Motorcraft plugs gapped at about .054, ran great. Smooth, all my lost power came back. Just my two cents.
As others have mentioned, stock spec is .054. As long as you are not blowing out spark, the bigger the gap, the stronger the spark. If you close your gap in to .045, you are essentially producing a weaker spark. But again, if you are using a stock motor, the difference between these will be minimal. On a boosted car, the gap is much smaller.