C30 heavy duty rear axle top speed

[Jscompto]

Hey everybody!
I had a question for those of you who have some good knowledge of these trucks....
I inherited my uncle's 1969 C30. It's a dump truck and it has a 350, a 4 speed manual transmission (including Granny gear) and an 11,000 lb rear axle (I assume it was a cab and chassy from factory). It's in pretty great shape and it all works too.

My question is this...what is everyone's consensus on top speed for these trucks?

The truck will only go about 40mph tops...it seems like it needs a 2 speed rear end in it or something because it is super slow. It will haul and dump whatever you put in it but it is just a snail on the road. I would have thought it would at least go 55-60mph...but no. I attached a few pictures of it as well. Can anyone help me figure out why it's so slow? Obviously I'd love to change either the rear end gearing or the axle out completely but I also don't want to change a whole lot of stuff on the truck. Thanks in advance!