No reason you cant. It's much less expensive to have a hard line loop around like that than, say, a tube and fin dedicated cooler. For the most part, if they did it from the factory, it should be fine for you. Of course, there are definitely exceptions..
I tell everyone to run them.. I run a Permacool 10"x 7" tube and fin right behind my intercooler.
big reason power steering systems fail is because of debris and heat. a cooler would help, too large of a cooler will mean longer warm up times (for trans at least, harder shifting) you spent all that money on the swap might as well go for a cooler of some type :P keep in mind earlier p/s 'coolers' consisted of a line looping, sometimes not even going all the way to the passenger side, eventually going all the way, some factory setups have fins built onto the line for other manuf's. dont trust what 500 miles shows for people this is a longevity issue.