Elements centered in a rectangle (border) element

Hi Everyone, ok I am wanting to copy and paste the elements from an old Illustration to a new one. It's pretty straight Forward and is almost complete, except for one problem. I have to copy and paste each element set seperately, as each element set has it's own layer:

Callouts

Identification Number

Raster Images

Vector elements

Border

and so on...

I am using 'copy and paste into place' ensuring all element positions remain in relation to eachother.

Question

How do I move the new elements from there pasted Position into the Center of an existing rectangle (Border). Note this rechtangle can different sizes and locations.

Many thanks in advance!

Alan