Anthony: I just finished building a tire rack for my open trailer. Here are a couple if thoughts from my experience.
1) use square tubing - clamps flat in corners and against side of the trailer, easier to clamp, easier to weld.
2) borrow or rent a hand held band saw to cut the tubing - makes great neat cuts, much faster than cutoff wheel or cutting torch.
3) keep the design simple - no need to bend tubing.
4) measure twice, cut once.
5) rotate the bars that will go across the trailer 45degrees - tire tread will rest against a flat surface rather than a corner
6) I used 1.5" tubing for the majority of the rack, but I needed slightly smaller tubing (1.25") to go through the hub holes of the wheels. Excess from the hold down bar fits inside of the larger tubing, made a great bracket for the hold down that can be adjusted for different size tires.
PM me with your email address and I will send you some pictures.
Where are you located in NYC? I grew up in Queens.