getting ready for #1

im getting ready to start my first one have my hf trailer going with the generic benroy profile just 4 feet wide for this one, my first ? ,first of many, is when i make my floor i should put my crossmembers in the same place as the crossmembers on my trailer right? and should i fir out where the crossmembers are so they are the same as my side rails so i get contact all the way around,,,thanks