The easiest way to register that traileris to say the trailer is assembled. No matter what it started as, whether its scrap steel or a 53' van, after you're done with the conversion, wouldn't you say you built it? There won't be a vin number, and the year will be the current year. Here in michigan, they don't ask to see it or anything. You'll have to weigh the trailer at a truck scale and tell them what it weighs, or some people just guess at it. (A great dane 53' van with sliding tandem axles is registered as 14,000lb, if you build it that way and want to guess
)
Oh yeah, be really careful what you call it. They'll print whatever you say on the registration as the make. I have a utility trailer that says its a 2003 homemade trailer, do you really want it to say that?
Also, anything over 2000lb in michigan has to have a title too. Still not a problem, just an extra piece of paper and an extra $20 or so.