Engine RPM at Hiway speed

Hey all, I have a 70 Duster 340 auto with 3.23 rear sure grip. At 60mph, the tach reads 3500 rpm. This seems high to me as I used to have a Dart w/340 and 904 auto and 3.23 sure grip and it would rev at 2800 rpm at 60mph. Could the 3rd gear ratios in the trans be that much different between the 727 & 904? The tach was rebuilt last year and the engine sound at 60 seems to match what 3500 rpm sounds like. Any thoughts?