The M110A2 was a later version of the M110 and the A2 was not used in Vietnam. The original M110 with its short barrel is the correct version for Vietnam. The major difference in the kits is the barrel. I don't know anyone who has the short barrel as a separate item. You may want to look at picking up an old M110 kit on ebay.
Eduard makes a nice photo etch set for the M110. You can also look for AFV club 155mm/8inch ammunition set.
Here are some other details setting up artillery.
http://www.redleg2scale.com/RBMI%20Info/Artillery%20Modeling.pdf
Rounds Complete!!