Hey, @Carlos G lives in NC. He registered his 550 spyder. Maybe he can help with guidance!!!!
I had good luck going to my local DMV and telling the "greeter" that I was trying to do something out of the ordinary (with a short explanation) and could I speak with either a supervisor or the Officer in Charge of that office for guidance (this was after I had read the procedure a bunch of times on the Mass DMV website and outlined it to understand it). THAT got me the name and number of the head of the Titling office and HE was a hidden gem. Straightforward, no BS and told me exactly what I needed to do, in what order, with what backup info, and the names of a few other people who knew how to get my change done from a VW sedan to a 57 Porsche Speedster Replica - some were in Boston and others were out closer to me mid-state.
In the end, I worked with four different offices for different stuff: Local office for new VIN paperwork (went through two people there until I got a supervisor who had last done one eight years ago), State Police two towns over for the VIN inspection and actual sticker applying, State titling office (Boston) for the title and registration changes and finally the emissions inspection (done by a non-government private company in Mass. one town over). After all that, I STILL needed to do my yearly inspection, but my emissions waiver was already "in the system" and I breezed right through.
In the entire state of Massachusetts (7 million of us "Massholes"), they often see less than a dozen of these a year (2018 had exactly three and I was one of them) and believe me, NO ONE at most DMV offices have ever processed one of these (in ANY state) so that is why you're getting the run around - they simply don't know what to do. So they try something and if it works they don't see you again, but if it doesn't then the next step catches you/him and you have to regroup and go back a couple of steps and start over.
To get around that, find a supervisor (you will need an experienced supervisor to do this conversion) who can outline the steps to take and, for each step, ask if he or she can recommend a person and contact info to help you along. Be polite at all times, impress on everyone that you're just trying to do the right thing and ask for what paperwork (some yours and some from the DMV) is needed at each step.
Remember that in one of the inspections (the State Police visit for me) they'll be looking for stolen parts used in the build so you may ( most likely ) need proof of ownership (bill of sale, invoice, etc.) for major parts. Ask what they need for satisfaction and then show them what you have or ask for alternatives, like "I don't have an invoice for the engine, but here is a Bill of Sale for the entire car", if that's all you can get. They'll be looking for proof of ownership on the engine, transmission and original car unless you bought the car already built and completed, then a bill of sale or similar should suffice.
Don't offer too much information - provide exactly what is asked for, no more, no less.