Taller Tires Needed

My runner with a factory 4.10 Dana and 28"tires is 3700 at 70 mph.
With no speedo, you might have been going faster than you think..... or, it doesn't have 3.73.... or, the converter has a bunch of slip in it.
this is my guess.

the maths don't pen out or everybody there is a lead footed speed demon.