What is this piece? Where can I find it

There is nothing wrong with the stock style linkage that is pictured (solid metal rod) but in case you can not find the appropriate rod (which was MY case) I installed the Lokar universal cable on my 360 swapped dart and I have been very happy with it. When I got it I got it with the combo Lokar throttle cable as well. Sometimes it's just nice to know that you have options out there but you should definitely have something connecting your throttle to your trans so that they can "communicate"
Good luck!