Making it a better driver

I have a 77 dodge d200 (3/4 ton) that had no trouble running 80 with a 16 trailer behind it. At that time the /6 in it had over 300.000 miles and showed no signs of having been opened! Later, due to a cracked vacuum booster hose, I burned a hole in #6 big enough to stick my thumb through. It was a dog getting up to speed but it would run 65-70 PULLING THE TRAILER UPHILL! 80 mph in a sedan with a /6 should not be a problem if it is tuned right.