J Mack,
What areas are you talking about, since I sprayed almost everything inside. Just the floor, I'd say 1 to 1 1/2 gals. of the Sound Control and the Ceramic insulation seems to go a little less, so buy 2 gallons. Same for the roof. Doors and tailgate, another gallon of SC. I didn't use the Ceramic in the tailgate, but I did in the doors. Also sprayed the firewall and up under the dash, really deadened the sound down.
I put down 2 coats of each, on all of it. I bought mine at a hotrod shop, so I could pick it up when needed. If I were doing it all at once and had to order, then maybe 3 gallons of each. The stuff 's not cheap, so buying as you go seems better.
Tip: Don't go heavy at one time, better to build up. it will crack as it dries, if too heavy. 45 min to an hour between coats, if warm. Dries slower when cold.
Hope I didn't confuse rather than help.