Transmission Question

[eglide78]

I need some "advice". I scored a 1971 GMC Custom Camper 2500 Series with 46K original miles and all original. On the glove box door it says M49 Turbo Hydra-Matic transmission. As I am going down the road I can feel it shift from what I presume is 1st... to 2nd(very smooth) to 3rd at around 30 MPH and it feels like it should shift once more at around 45 - 50 but it does not. Doing 55 MPH it seems as though the engine is pulling some substantial RPMs... should it be shifting one more time? What would keep it from going into that final gear... The truck runs and drives great except for this issue. It was bought brand new and when Grandpa passed he left it to his son. When his son couldn't drive anymore he turned the truck over to his daughter and that's who I got it from... she had it about 10 years.
Any assistance on this transmission shifting question will be greatly appreciated.
Thank you