Good question - I dunno. If it's like anything else, there are probably a lot of changes, with little that are outright glaring, except to the trained eye. Here's my "4 Rules for Modeling Sanity, which may help:
1. Absolute Perfection is un-achievable in miniature. You can get close, but that's it.
2. Critics always abound, the worst of which is yourself. Relax. This is fun, remember? Have a beer.
3. Few people, if any, will ever notice a missing part, paint goofs, or the Nth detail you didn't add.
4. If by a random act of chance someone does notice, simply shrug and refer them to Rule #1.
With that being said, could you build it as a "T" model, instead? Here's what globalsecurity.com says about the "Tango":
"With increasing demands for higher performance, particularly greater load-carrying capability in high temperature conditions, Bell developed improved dynamic components for the Huey series. Application of these components, which included a larger diameter rotor, led to the 309 attack helo in the early Seventies. This allowed an increased payload, providing more combat capability. The subsequent Marine-ordered version of the King Cobra was designated the AH-1T.
The United States Marine Corps (USMC) had identified a need for more armaments; thus, the AH-1T upgrade was initiated. This aircraft had an extended tailboom and fuselage and an upgraded transmission and engines. In addition to the modifications for improved combat effectiveness, major efforts were made to incorporate the lessons of the Cobra experience in achieving greater reliability and maintainability. With the TOW missile system added to its weapons, the AH-1T gave Marines a ground attack capability far beyond that first envisioned by their predecessors who took the first Marine Huey Cobras into combat in the late 1960s.
The AH-1T is fully capable of performing its attack mission in all weather conditions. Additional missions include direct air support, antitank, armed escort, and air to air combat. The TOW missile targeting system uses a telescopic sight unit (traverse 110º, elevation 60º/+30º), a laser augmented tracking capability, thermal sights and a FLIR to allow for acquisition, launch, and tracking of all types of TOW missiles in all weather conditions. The Cobra also uses a digital ballistic computer, a HUD, Doppler nav, and a low speed air data sensor on the starboard side for firing, and has in-flight boresighting. External stores are mounted on underwing external stores points. Each wing has two hardpoints for a total of four stations. A representative mix when targeting armor formations would be eight TOW missiles, two 2.75-in rocket pods, and 750x 20-mm rounds. The gun must be centered before firing underwing stores. Armored cockpit can withstand small arms fire, and composite blades and tailboom are able withstand damage from 23-mm cannon hits.small arms fire, and composite blades and tailboom able to withstand damage from 23-mm cannon hits."
Heres a nice build review of a USN/USMC AH-1T, 1/72 :
http://modelingmadness.com/reviews/mod/dorrah1t.htm
A small blueprint:
http://www.the-blueprints.com/index.php?blueprints/modernplanes/helicopters-a-b/5327/view/
All in all, the Tango looks like a Cobra to me. I say build up the Tango, as is.