Hey Rob, glad to hear you got all your bits together!

You do need to cut out pretty much exactly the amount that the insert is shorter. The only thing you need to be careful of is that the cap seats on the insert the same way. As in if the flat of the cap touches on the same part on top of the insert. The Stanza cap has a ring inside that holds down the insert. With the R31 inserts being bigger in diameter don't touch the ring so it makes it seats a bit different. I think the 180 strut caps are flat so shouldn't be a problem. Just put your tape measure down the tube with the insert in to make sure of your measurements.
My struts are cut up ready to be welded hopefully tomorrow. I'll take some pictures of that and the inserts and how they go into the strut tops.