Personally i would go w/the 3/4. You might do a little research on what type of plywood to use (species and pressure treated or not). If i understand correctly newer pressure treated wood uses a sodium solution of some sort won't be very freindly to the metal. (Someone else may have more info on it, but everything i have read regarding trucks beds is to not use pressure treated.)
http://www.jalopyjournal.com/forum/t...active.463308/
http://www.stovebolt.com/ubbthreads/...&Number=364835