Why is my top speed 70 mph?

Just be concise with what you checked. Like
a. Jacked wheels up, turn freely.
b. No cat, pipes are damage free, exhaust flows freely.
c. No animal, vegetable, or mineral material wrapped around drive shaft.
d. Pulled trans dipstick, oil is full, clean, bright red, and free of burned smell.
(We can worry later if it is the right stick.)
e. Pulled fuel line at carb, full flow is satisfactory.
f. Right rear donut tire is properly inflated.
There are many good suggestions here. If you skip some because you may not understand, or say lack a specific tool, tell us that too. Once again, we are not there. I am hoping that you are able to resolve this, as I would like to know how it works out. So many times these threads die when the OP figures it out and the rest of us never know what happened. Have at it, don't let the car win.