heater swap options

My heater leaks to the feet of the passanger, fan makes noise especially on higher speed setting and someone has damaged the fan enclosure while cutting metal from engine bay when trying to remove heater that way (without success of course). I don't think this heater is worth of fixing. However, GT heaters are unavailable in Finland and at the moment I'm not (yet) desperate enought to order working used heater from Germany.
Has anyone ever done heater swap from another car (or maybe even another Opel) to GT? I understand it won't be a bolt-on swap and I'm ready to do minor welding, etc. I would also be glad if I got a bit more room for my dual carb setup so I could fit longer trumpets (1.77", 45mm) and intake airbox. I know these can be made to fit even with stock heater, but more clearance wouldn't be a bad thing.
My girlfriend isn't terribly tall so maybe it would be possible to fit some kind of heater into the ..'feet room' if everything else fails. Anybody done that?
Of course there is a lot of room for heater in the rear of the car, but that doesn't help much if visibility through windshield is near zero.
All ideas are welcome! Fall is approaching...