Why is my top speed 70 mph?

I doubt gearing is a problem or it wouldn't be racing in the moped class. I bet it's a 318 in a /6 car witha stock diff.
After following RRR advice, and checking the kick down, I would check the fuel delivery and put a fresh filter on.
If your mechanic didn't see anything wrong then he must be a Ford mechanic.
I would start by doing a thorough inventory of what you have- engine #, tranny, rear end and gearing, carb, distributor etc. get to know what you have so that you can build on your mopar education.

Lots of help on here and if you don't know- just post a picture and someone will tell you what it is