Jiminy Kriket

The holes are threaded, yes? You could get away with silver soldering threaded rod in there and then redrilling the pattern to suit.
The biggest problem you are going to have soldering or brazing the plug in Gerg is getting the bell housing clean enough down the threads before you
apply the solder or braze. Silver soldering would be preferred because there is a lot less heat involved in melting the solder. Silver solder is also
strong as hell when used this way. Just make sure you get the high silver percentage solder, not the plumbers solder. The higher percentage solder has
a higher bond strength.
The only steel bell housings i have seen are the pressed metal jobbies sold for drag racing applications like a lakewood and the like.
Other than that, you only have cast iron and aluminium as options for stock bell housings. 
And on a general note on welding/brazing/soldering cast iron, preheat is your friend. Get it hot enough so it's "smoking" before you try to do any repairs.
At work i use a 4 ring camp burner to preheat larger castings But at home, if it fits and the wife isn't home, into the oven at 150 for about half an hour before
I weld then back into the oven for another 20 minutes after I finish at 150, then leave it in the oven to cool off. This just stops any possible heat spikes that
may harden the cast iron or worst case scenario crack the housing.

Hi, first post but I hope I can help.
I'm a fitter/machinist by trade and in the past 5 or so years have converted 6 bell housings to suit the T5 gearbox bell housing.
Fist things first, is there enough material at the bottom of the bell housing to catch the bottom 2 holes on the gearbox?
If not, there needs to be a couple of bosses welded onto the bottom of the housing to get the holes in the right place.
Second, do you have any holes interfering with the holes for the T5 bolt pattern?
If there are overlapping holes, my fix for this is metal spraying up the original holes. Metal spray is an oxy acetylene welding process
that doesn't cause the same hardness or warping issues that stick welding can cause. 
Thirdly, Does the locating diameter on the T5 snout fit neatly into the bell housing already?
If it's too loose, the snout will need to be sleeved to fit. If it is too small for the snout to fit, the easiest fix is to skim the location diameter
on the snout down to suit the bell housing. You can bore out the bell housing to suit, but it takes longer set up.
Once you have all the necessary lugs and available material for the bolt pattern, it's simply a case of drilling and tapping to suit.
 
The cars I've converted include my 302 windsor powered xy, and 4 crossflows in xes and xfs. Biggest problem I found
was that the bell housings started to taper too high up and you couldn't physically get the bottom 2 bolt onto a flat surface, but as I said,
I have worked around that issue.
Hope this helps, if you have any questions on the welding process, just ask.

Hi, first post but I hope I can help.
I'm a fitter/machinist by trade and in the past 5 or so years have converted 6 bell housings to suit the T5 gearbox bell housing.
Fist things first, is there enough material at the bottom of the bell housing to catch the bottom 2 holes on the gearbox?
If not, there needs to be a couple of bosses welded onto the bottom of the housing to get the holes in the right place.
Second, do you have any holes interfering with the holes for the T5 bolt pattern?
If there are overlapping holes, my fix for this is metal spraying up the original holes. Metal spray is an oxy acetylene welding process
that doesn't cause the same hardness or warping issues that stick welding can cause. 
Thirdly, Does the locating diameter on the T5 snout fit neatly into the bell housing already?
If it's too loose, the snout will need to be sleeved to fit. If it is too small for the snout to fit, the easiest fix is to skim the location diameter
on the snout down to suit the bell housing. You can bore out the bell housing to suit, but it takes longer set up.
Once you have all the necessary lugs and available material for the bolt pattern, it's simply a case of drilling and tapping to suit.
 
The cars I've converted include my 302 windsor powered xy, and 4 crossflows in xes and xfs. Biggest problem I found
was that the bell housings started to taper too high up and you couldn't physically get the bottom 2 bolt onto a flat surface, but as I said,
I have worked around that issue.
Hope this helps, if you have any questions on the welding process, just ask.