my swb has 750 tyres and fairey overdrive. in 4th it will sit at 60mph (actual) with no issues with more left in reserve if you were mad enough, revs are a too high for my liking though (more so if it were a diesel) with over drive engaged it knocks the revs down nicely and will still accelerate believe it or not (on flat of course)
at 70 in OD the mixture of aero/engine/drive train noise just doesn't "feel" right, i never do it, but just wanted to share my experience and that do not agree it swamps the engine.
like most folk i only really use it as a 5th gear, tend to engage it anything over 40 for pootling along. or the other times when deliberately boring people to death about how its now got 16 forward speeds then unnecessarily (and smugly) going 1st, 1st OD, 2nd, 2nd OD