Which to install 391 rearend gear or lower?

P245/60R14 would be a hair shorter than a 26" tire, 25.6" by my math. 3.91 gears would be a bit low if you plan on extensive highway use. I would go 3.55. If you stay mostly on back roads or take it to the track more than once or twice a year, 3.91 would be my choice.

I'm running 4.30:1 gears in mine with a 28" tire "275/60R15". Great around town, back roads and on the track, but I won't take the car more than a few exits on the highway. Further than that, it goes on the trailer.