With that combo you should be doing 60 mph at 2100 rpm's and 45 mph should be around 1580 rpm's.

Either your speedo drive pinion is incorrect (should be a 27 tooth) or your speedo or tach is incorrect.

Also, high stall converters and low numeric ratio rear ends often don't play well together, you might be experiencing some converter slip or a combination of all of the above.