Extrusion in Inventor

[Madison]

Okay - keep in mind that this is the first time I've touched Inventor, practically. I've never done anything useful with it. I'm an AutoCAD kid.

I've downloaded the Bosch 30x30 extrusion part from Ed Sparks' website (www.firstcadlibrary.com) and I'd like to use it to experiment with chassis designs.

However, when I go and try to do this as an assembly, I seem unable to individually affect pieces of extrusion.

That is, it I wanted to make a rectangle that is 30" x 36", I can't.

I insert a new part four times, each time choosing the 30x30 profile. Ed's part is extruded to one inch. Each time I edit the part, and then try to edit the extrude feature, I am able to change its length. However, all four pieces of extrusion are affected. I want to be able to change each instance of that part to a unique length. Is this possible?

It seems a little ridiculous that I'd have to go in and create a new part for every unique length of extrusion. I wouldn't mind, even, saving each unique length as a separate *.ipt file, but there must be some way to change them individually within the assembly mode.

Any ideas?