I agree with all above but also you state you are running 14" wheels, I am too, and I am so sorry I bought 14" mags years ago and not 15", there's not a lot of tire options for 14", I am running my stock muncie with a 3:42 rear, before I had the rear end rebuilt it was a 3:08, am I glad I didn't go higher than that, as it was I found the tallest BF TA radials 14" I could and the rear tires are just over 26" tall the last set was @ 24" and what a huge difference, I was over 3grand on the hiway and am now @2500-2800 so it's doable. I changed my speedo gear in the tranny after doing the calculation and it's about as close as factory for mph.

Even with the 5sp with a 3:90 you might want to increase tire height, but see how she runs.