Dusty-
This is an issue that should be one of the first things a potential buyer thinks about, but is often an afterthought.
The short answer to your question is "every state is different". You'll need to get on the Colorado DMV (or Sec. of State, or whoever is in charge in CO) website, and do a complete search for "replica vehicles", "street rods", or related vehicles. Almost every state has a "special construction" title- but the year of manuf. usually matches the year the car was fist titled as such, or at best, the year on the Certificate of Origin assuming your car came with one.
I'm not a lawyer (far from it), but would advise you that neither are any of the various builders or title services that advertise in "Kit Car". Just because VS or JPS or TR uses the VW VIN and title, doesn't necessarily make it legal in your state. Further, just because some title service in NY or GA says they can legally re-title you as a '58 Porsche, also doesn't make it legal. (I've done both of these things). Again, I'm no lawyer, but I'll tell you what- the definition of fraud is to represent something differently than it is.
Illinois has a program to legally title and register limited use replica vehicles in the year they are built to represent. It was a provision pushed through by the street rodders in our state, who were being harassed by over-zealous law enforcement over title and registration issues. My car is legally titled, registered, and insured as a "1958 Intermeccanic (no room for the "a") Replica". It took a bit of hassle, but now I'm in no danger of my car being impounded. BTW: I payed my sales tax on the entire purchase price.
I know CA has the magic 500 BAR program. I'm sure CO has some provision to do what you want legally. A lot of states (mine included) have no emissions inspections, so the point of seeing a 1957 or 1958 on the title is moot- unless you ever want to sell the car, at which point you are limiting your potential market to people who live in "no inspection" states, which eliminates CA (far and away the biggest market for these cars).
My advise? Think really hard about it before you get all hyped up about a "1957 Porsche" title. If CO doesn't have what you are after, you may be able to use a title service to retitle as a "1958 special construction" (or similar), which would have been what I would have done had I kept either of the first two speedsters I owned.
Good luck.