It's 100% legal.
The Montana registered agent registers an LLC (a business) for you, with you being the owner. The agent provides you with a mailing address and acts as an in-state proxy for you. You then sell your car to your LLC, to which it is then titled. It is then considered a company car and can legally be registered in Montana. Since you own the LLC that owns the car, you can drive it anywhere, as long ad you carry insurance on it, and are a state-licensed driver.
Insurance is trickier. My daily driver insurer laughed at me when I mentioned the Montana registration, then laughed harder when I mentioned it being a kit car. But I came across a specialty insurance agent who painlessly got it insured through Modern for less than $500 a year. I since spoke to Modern and was assured that the Montana registration was not an issue, and they do it all the time.
Disclaimer: I have not made an insurance claim, nor have I been ticketed, so if/when that hammer drops, may be up ****'s Creek with no paddle, who knows.
For what it's worth, I've had multiple encounters with police, and not once has one given me trouble. Most of them offer a thumbs-up and I even honk and wave to them now. I respect them though, I dont speed or act stupid on the road.