Most of the 79's with 403 used the regular housing #409037 and a different top radiator hose, and it fit like rubbish and looked just as bad. The better early housing is 77-78 Firebird specific with Olds so hard to find as you've seen. There was a NOS one on ebay a few weeks ago, but listed for money it wasn't worth also.
No, the pipe has been cast into it so there's no cutting it out to reuse. You'd still have to cut off the pipe and reattach with an inserted tube for strength before welding up. I repaired one by tig welding a few pinholes but otherwise not worth playing with. Depending on where the corrosion is you could fit an earlier Oldsmobile rubber J-hose as a sleeve over the tube curve, not just the end gap. They do still turn up, in various conditions, not always described at thermostat housing, water neck is common, rarely mention part numbers either.