When I built my last 5 x 8 utility trailer my brother was a head of sales at a large lumber supply around here and he got me some 1 1/8" plywood subfloor to put on the deck. Man, that stuff was heavy but it is incredibly strong and has held up amazingly well for many years. I just soaked both sides with a sealing oil and let it go. Looks like you have a lot bigger job than my small trailer but the work looks good.
For my spare tire I found a mount at Harbor Freight that I mounted on the side of the trailer to get it out of the bed of the trailer. Works great and is easy to access if I never need it- knock on wood I never will...
