change up - change down or leave

[T1 Terry]

The engines can rev to 95km/hr ,or more, in 3rd so 60km/ hr is no sweat.
Too lazy at mo to work out the appropriate revs
As you noted, they are air cooled so pick revs over lugging anytime.
I sometimes have it in 4th @ 60km but any hint of a upward slope i change back to 3rd.
Less strain on engine in my view 

Thats for 1800 and 2 litre. Stock ish OD tyres.

My '74 kombi has an 1800 motor and 2 ltr gearbox. The diff gearing is a bit higher in the 2 ltr gearbox and 60km/h in 4th is for flat running and slightly down hill. 
Not only is the cooling less at low revs, so is the speed the oil pump is turning. Not so much on an issue with a newly build engine, but in a well worn engine it means less oil pressure. Lugging also destroys bearings, excessively high revs scuff rings, but the 1800 revs happily to 5,500 rpm and that takes my kombi up to 100km/h in 3rd, though I don't generally do that rpm to it unless it's been running for a few hrs and well warmed up and only so I can snatch 4th on a slight uphill climb or just as I crest a big hill to get away from the B double that will appear in the mirror on the next hill 
I'd estimate 80km/h in 3rd is well within the rev range that the engine is happy to spin at ....

T1 Terry