desubguy,
As others have suggested in different ways here... Yes - doing your homework is the key whether you build your own or buy a new or used Cobra replica.
Not to rain on the parade, but there is no guarantee at getting an SB100 number. The ones that got them last year will tell you it was relatively easy. For those who did not get one, they were not happy and waiting in the queue for next year again, and there were more than a few. The key is making sure your local office knows the procedure and they are ready, and yes, getting their early. This last go-around at DMV for 2006, by 11:45a.m., the queue of requests from each site had caught up with processing numbers at headquarters and all 500 were gone and allocated to all the lucky ones.
CAUTION / All:
And, if you show up to DMV on January 3rd in 2007 as suggested by others here (I've seen it stated more than a few times on Clubcobra lately), its possible won't get a number for sure. Last year, Sunday was a holiday and DMV considers the next day a holiday if the Jan 1 holiday falls on a Sunday. In '07, the Jan 1st holiday falls on a Monday, so its possible they will open on Tuesday Jan 2nd at 8a.m; NOT January 3rd.
If you buy a new kit to build or a used car and need that SB100# in 2007...
1) Get that MSO or title before Dec 31st 2006, in your hands!
2) Check those dates and be sure to show up at DMV on the right day!
Good Luck.