I hope this helps...
I 've worked on both SH3G and the S61L/N and there some differences in the machines Vs their Westland counterparts.
To start with you're going to need at least two SH3 kits as the S61L/N is longer in fuselage than it's military brotheren. This is most noticeable between the engine inlets and the cockpit. All doors are on the right side of the S61 with the cargo door being forward to just behind the cockpit...leave room for the AUX SERVO compartment between the cockpit and the cargo door. (Rough dimension would be the width of the air stair.) Speaking of which, it is located aft where the cargo door begins, again on the right side. Twin pitot tubes on the SH3 are on the roof aft odf the cockpit and fwd of the inlets. On the S61 they stick up on a mast through the "Beanie" as shown on your pictures.
Word of note here a device called a "Bifilar" sits below the "Beanie" but above the rotor head, "Bifilar" was only use on the S61 and not on the SH3. Also delete all blade fold mechinisums and lines for S61, as it used an Air Force stile rotor head. (Some Air Force Heads used the "Bifilar" also)
As for the "rotary rudder" (tail rotor) Westland Commando had a 6 bladed tail rotor while the U.S. counterparts had a 5 bladed version. The Commando may be a good source of spares for 1/72 application because of the landing gear.
Landing grear SH3G and S61N both use the round stile floats/sponsons they can be the larger big stile or the narrower style with the floatation bags on the outboard sides of the unit. The S61L is the only version to use the sponsonless "L-Gear" ( In the Commando kit). For 1/48, Belcher Bits makes a nice set of the narrow sponsons with the correct inflateable floatation bags.
Lastly we come to the vertical fin, the S61's is longer and taller and sits at a steeper angle than the more graceful SH3 version. (I want to say closer to 45 deg, but that may not be correct)
Oh, and both machines may/may not use the rescue hoist... Fun huh?
Able Audacious Army Aviation Above All!