It should be able to drive any posted speed limit all day long. 65 would not be a problem at all. Did you check the transmission fluid level? It could be low.

@tiny 

How’s the best way to check transmission fluid? 
engine running?

engine running, transmission in drive?

engine off? 

@bruce-bernhjelm 

Drive until the transmission is warmed up,

engine running,

in Park.

Tiny's diagnosis is the most likely problem.

We had a Canadian 1964 Pontiac with a Powerglide that actually had to be kept 1 pint over full to shift properly...

right from the time it was new.

Mark this day on your calendar but I'm going to disagree with Ole. 😀 You don't check the early PG in Park. You can check it hot or cold but you check it in Neutral at idle. If you check it in Park the pump is not working. If you check it hot it will read higher than if you check it cold. If my 53 is above the add mark cold I don't add fluid. If it's at or below the add mark cold I add fluid until it's about half between add and full. If it's hot I add to just under the full mark.

https://vccachat.org/ubbthreads.php/ubb/showflat/Number/438853/Searchpage/1/Main/68422/Words/Powerglide/Search/true/re-powerglide-question.html#Post438853

https://chevy.oldcarmanualproject.com/shop/1949_53/00lube/0_5.HTM