basically if you go with collector plates (25 yrs or older), you hafta get emissions, and pay for your tags for 5 yrs, and pay again every 5 years without having to get a new emissions test each time. if it is super old (40's - 50's), you don't even need emissions.
now if the vehicle changes owners, i think you hafta get retested.
also, most shops can't (won't) legally try and get you to pass the test. you just hafta search and find one that will. the price has also gone up to $25 per test from $15.
last time i did emissions was on my '85 f150 after i had swapped intake, carb and exhaust and a bunch of emissions junk.
i had Thor at Thor's Automotive (off nevada south of austin bluffs/gog) tune it up for me and then i took it to Lube Stop (iirc) on filmore hill and they didn't even do a visual check (would have proly failed-even with smog pump and egr in place). it passed well under all of the requirements.