Why is my top speed 70 mph?

Looks like there consensus that there's something wrong.
I would step back & first determine if it's low engine output or something downstream in the driveline.
So first I would check engine stuff like getting full throttle, fuel filter, ignition timing, exhaust back pressure, clean air filter & stuff.
If nothing seems really out of line with the engine start checking trans stuff, burnt fluid (indicates trans issue) & kick down linkage.
You also might want to check for brake drag.
A vacuum gauge on a full throttle fun might tell us a lot too.
But if I had to make a wide guess I'd say it was a trans/ torque converter issue.