Drivetrain / Rear-end question

[cpnbnanamn]

So, I'm trying to understand why my truck doesn't have a lot of speed. I've got a '67 C10 SWB with the 250 cu in. inline 6, w/ 3ott. The rear-end looks like it has the stamping of HA, which by all research, should be a 3.73 ratio, right?
When I head down the road, I get it up to 3rd gear, and I can't get to any more than about 55-60 MPH without revving too high.
I'm pretty mechanically savvy, and have been doing a lot of work on the truck already, and understand the basic theory of trans and rear-end gearing, but I'm by far no expert. Am I geared too high, or too low? I'd like to get closer to highway speeds out of it. I could use all the education I can get.
Eventually, I'm wanting to remove the 6 and trans, and put in a 350 sbc, and a 5-spd manual, but I'm not ready to do that, yet.