Car won't run past 60mph

Have you run the car with another engine/trans combo and it cruised okay?

I would narrow it to the rear gear or transmission issues. You may need to pop the pan and check the band adjustments, then top with new fluid.

Before you do that try and make sure that it is actually engaging third gear. I used to have a gearbox that had a damaged shift link and it did not like shifting properly until I put new links on it.

- boingk