It's amazing how much abuse these Fords will take and still keep ticking.
If it were me I would do one of three things.
Leave it as is and just run it - it might go for a long time. Or buy a different transmission and install that. You might even buy just an empty case and install the parts from the broken one into it.
It is not hard to do and a case could be purchased fairly cheap.
As for welding it yes it can be done. But welding cast is expensive and takes some skills.
Watch the slideshow at the link below for a transmission repair my pal Kenny and I did on a Ford 5000 transmission last summer.
A couple of added comments about it here:
It's amazing that that tractor ran for years with that hole in the side with no ill effects.
It was expensive - the junk case cost $100 and the rod we used was close to $100/lb and we used 2 1/2 lbs to do the job.
But, we did save an otherwise good tranny and rear end which he sold it for $1600 so it was worth the investment.