Interior dimensions: the width meets federal spec but the length is 1/2 inch too long at 53'1/2. Simply cut all your length boards to 53 instead of 53'1/2 if you like. If I were to do the crib again, I'd probably cut the length to 52'1/2 inches, as this would be a perfect fit for almost all mattresses. If you go this route, remember to keep the mattress support slats flush with the top of the support frame to give a level surface. The pictures indicate the slats being inset a bit.
What helped us:
Wood spacers to place between the rails during installation.
We wanted to be able to disassemble the crib, so we put some finishing screws on the side of the crib and did not use glue
I would add recommending buying krieg jig PLUGS. They look much better than wood filling all the holes and probably save time and money for all the wood filler you use and wait time for drying and refilling the gaps that happen when the wood filler dries.
I did not make the slanted leg supports but instead did a straight leg support. I was told that the the slanted leg design might not hold up over time because they are a weak point of the body and with normal usage they are susecptible to breaking.
For all the postings on federal law. The law applies only to cribs for public purchase. If you are building it for your own use. You can make the slats however far or close you want. It is only a requirement for selling.
Design confidential has a nice section on Lumber and The Raw Deal. Since I cut my own wood my length was nominal measured instead of the 'actual'. For example: 1x3's are actually 3/4 x 2 1/2. http://www.thedesignconfidential.com/2010/09/build-it-lumber-and-the-raw-deal
Because of this my slats ended up meeting the federal guidelines. It did cause some other problems for me, particulary with the mattress frame. See Josh's recommendation above.
Hope that helps out anyone building this.