Which to install 391 rearend gear or lower?

3:55 is the lowest I'd go with those short tires since you say it's mostly a hwy cruiser. I had 3:73's in my Cuda with 27" tires and it was revving over 3000 at 65. I put in 3:55's and taller 28" tires and I like it alot better. It's not as quick but not a tremendous diff and the hwy cruising is sure alot more enjoyable. Gained about 3 mpg too.

BTW: What cubic inch is the engine. Bored .030 over means nothing in the realm of things.