Just go down to the DMV. Tell them you built a 40' boat and would like to register it before you launch it next week. See what they say.
I have already registered my boat in Michigan as a pontoon boat although I have yet to complete it. What's a pontoon boat? The codes don't say, but do specify to measure the center deck
length. In my case my 34' cat is then about 25' on the papers and the pontoon rego is $30 for 3 years. When I transfer to FL, if I do, I will be paying on 25' which is a bunch in my county. About $75 a year but 34' would be like $200 a year. Also 25' is covered on most homeowners policies.
Will someone ever measure it? Doubt it.