4000 rpm at 60mph without overdrive sounds about right under the current configuration. Depending on the tire size, the wheels are turning about 890 revolutions per mile, so that would translate into about 3800 rpm at the driveshaft with a 4.3 rear diff. which is what you probably have.

What puzzles me is why a late model 1800 with manual transmission would not have overdrive already. I thought all of them had it. Perhaps a previous owner removed it, or an Oz-only model?

In any case, adding an overdrive transmission without changing the rear diff would drop your 60 mph engine rpm's to about 3000 rpm.