I did it for my '71 Challenger. It doesn't cost anything, just time and effort to fill out the form and send/bring it to the CO Motor Vehicle on 18th and Pierce. Be sure to include a photocopy of the plates. They will send you back a "permission slip" that you need to have in your car in case you get pulled over. Technically you can only have them on going to/from a show or charity event for the car. However, I believe everyday I drive my car is a charity event! 
The way I read it they can be used anytime as they are going under the same rules as a Horseless Carriage. I’m no lawyer but here is the current statute, I asked a traffic officer I know about it a few days ago. He had no idea what I was talking about, so I will be carrying a copy of the statute with my registration.
CRS 42-12-302. Original plates
A person shall not drive the vehicle bearing the original plates except as
authorized in section 42-12-301 (5).
CRS 42-12-301
(5) A person may drive a motor vehicle with the special registration plates
authorized by this section or section 42-12-302 on the streets and highways, but only:
(a) To and from assemblies, conventions, or other meetings where such vehicles
and their ownership are the primary interest;
(b) On special occasions, for demonstrations and parades;
(c) On occasions when the operation of the vehicle on the streets and highways
will not constitute a traffic hazard; and
(d) To, from, and during local, state, or national tours held primarily for the
exhibition and enjoyment of such vehicles.
