First of all .... welcome to FSM Lester.
There are aluminum gun barrels available for this kit from Brass Barrels.
Here is a link to one of the distributers for this product.
http://www.bentbutnotbroken.com/Barrels.htm
I haven't ran across any other detail sets for the 1/16 Sherman, mind you I haven't looked either, seems I have enough on my hands with 1/35.
You may want to check out this site, they seem to have a few 1/16 Shermans on it.
http://www.fragarmor.org/Gallery%20C.htm
Best of luck and enjoy the forums !!!
Happy Modelling and God Bless
Robert