I have the 3.77 gearing and I have not experienced any trouble at any speed in any gear and in any kind of traffic. I use my Pantera a lot and twice to France (from Denmark). Tell me what I should look for of problems..... I should mention I have a 393" stroker now but with my standard 351c I did not experience any problems either ever!
I can only recommend it and I usually only shift up to 3rd or 4th on country roads (80km/h) and 5th is a blessing on the highway and keeps the rpm's down to about 3000 at 150km/h