Something is way off here. You say that the tach and speedo are correct, but if this is in fact a 3.55 gear, with a converter that is relatively tight, 3500 RPM is out to lunch.
A couple of things come to mind:
The tach is set to 6 cyl.
The rear gear is more like 4.56.
The trans is not shifting into 3rd gear - it remains in 2nd.
Just for reference - a long time ago I had a Datsun pickup with a 4.38 gear and it would sit at 3600 RPM at 60 MPH, this with a 25" tire.