The length of the primaries determine the "tuned" RPM of the header. Shorties can work just as well as LTs BUT at a much higher RPM range and in a narrower band. LTs give a bump where you want it and where the cam etc. can use it and are much more desirable for a street car. Absolutely get headers coated as it is not an appearance or longevity mod but a performance mod. Coatings keep the heat in the exhaust keeping it out of the engine bay and keeping the exhaust gases velocity up.
BTW with LTs the passenger's header drops right in from the top. The driver's side on the other hand usually goes in better from the bottom, may need a brace removed or modified and needs the steering rack dropped. It's well worth the effort in the end though.