My only concern would be since they are designed to take the hit or damage should you encounter it. If you had to replace them or one, removal for servicing them or anything else under may become troublesome. I've know people who've done so [welded] and they never had a complaint, other's wished they hadn't. From many manufactures like Shrockworks, Rockyroad, etc they say you can weld them but often depending on who you talk to and the type of vehicle you'll get mixed feelings about welding them on.
I do know some vehicles we deal with in specifics like Ford Rangers and their C-frame, its strongly advised by many companies to not weld on the frame as doing so hampers the metals ability to control flexing properly [despite ours and thousands of others argument of a suspension should flex not a frame, it works for the Ranger platform] and oddly has an affect on the metals compensation to fight off corrosion aspects by altering the metal heat treating. I don't think Nissan has such an issue or process that would affect their boxed ladder frame setup, just pointing out some things become worse.
Just food for thoughts.
What brand are you running on your X Prawn?