I just installed a vinyl top on my 70 Challenger. When I bought it from Legendary they told me it was the hardest top to install, they were right.
The previous top someone cut the top below the back glass to get it to fit.
I found that if you glue the area below the rear glass to the body line above the back glass before you do the c-pillars it makes a easier.
As was said it is very important to center the vinyl top to the roof and a heat gun helps. Also don't try to do it yourself.
Good luck!