Chrysler recommends a big, expensive tool that fits completely over the top of the upper spring mount, but it's not practical for the DIYer. There are other types like this, for example:
that come in from one side only and may work better for you. Another possibility is to take the parts over to a shop that does a lot of springs and struts and ask them to assemble the strut for you (for a fee, I'm sure). They have different equipment that is wall- or floor-mounted and that is powered by air or hydraulics and is very quick.
Let us know what you finally decide on and how it works out.