Didn't some of the later '70s and '80s B/F/M/J bodies have 2.45 gears and 318s? I may be out in left field here, but seems like with a big block it would be a non-issue.

For many years I had 2.76s and a mild 383 in my 3800 lb RR (with a single plane manifold, no less) and it didn't seem to bother it a bit. Don't remember the rpms, but the math says 2250 rpm at 60 mph. Even with the 383, it felt like it needed another gear. (Maybe from all the years driving the DD Toyota 5-speed.)