Did My Wildcat , about the same size. I used 2 large boxes including the roof. Just a heads up I purchased a hard rubber roller on ebay. It made installation way easer. The free roller you get free in some boxes is not that great.
I used 2 boxes for my 68 Skylark ragtop. It took it all to do the floor pan all the way from the dash panel to the top well. I don't know what size boxes I got, but I got them from Summit Racing. Very pleased. The carpet even fit better after the installation of the Dynamat.