The real answer is defined mostly by where you live. The state/city you live in has specific laws (In Colorado, it's by County).
For example, Denver area requires emissions, but it's emissioned by the year of the engine. I'm helping a buddy build a CalSpyder (Ferrari Daytona Spider replica - http://www.calspyder.com) and as long as you pick a pre-emissions engine (72 or earlier) you only need to meet the specifications for that engine (i.e. a charcoal canister). However, most emissions shops won't go into that far, depending.
If you pick an newer engine (say, 1983) then you are required to have catalytic convertor(s) - and that's something that an emission shop WOULD see.
So, what area do you live in, and perhaps someone from that area could reply. There are also a lot of good online resources that the States put up regarding such things. (try yahoo - government - state, or do a google search).