Rubicon already make a Stug III G kit , number 280017 ( 2015 ) , with the StuH barrel plus 3 different roof options , Pig's Head mantle , add on front hull armor plates and 2 MG shield types , a very nice kit although the lower nose plate angle is wrong IMO . An early short barrel Stug III would probably go over well with the early war gamers . I've often wondered why Rubicon didn't mold all the panzer IV lower hull , track , wheels etc on one sprue , then it would be so easy to just make a new upper hull / gun sprue or two for various panzer IV types like you mentioned . Dragon seems to do this a lot in their various 1/35 kits , although you do end up with lots of extra parts left over , not a bad thing if you like kit bashing IMO