Does anyone know the square feet of the interior of a 1983 El Camino? i want to dynomatt the whole interior but i need to know about how big of a piece i need..

Don't know off hand, but should be easy to figure out. Just measure individually the length and width of the floor, roof, back box, and doors. Multiply length times width for each section and add them all up.
Let us know what you find and save us the trouble of doing it. Also add 10 percent more for error.
Mike