I recently did this to my thermostat housing, I just got a alloy welder to weld it back on after I had cut the tube bit off the dome housing part and gave it an angle, it is hard to weld though apparently, getting a weld without inclusions is pretty hard, the welder had to clean, clean and clean it and ended up doing it twice because of porosity problems...
I should of taken a photo, make sure you either get them to put a bead of weld around the hose outlet so there is something to help hold the hose on or use the original end bit... I actually ended up makging it a tad shorter as well to make it fit nice, the rad hose has a tiny bit of length to give.
do a search for RIPS plenum for RB25 they use to include a modified thermo housing as part of the kit...