I would think drilling and taping that weld would be much more of a challenge than doing it in unmolested cast iron. My luck would be a broken drill bit or tap, or both.

Today's update. I have spoken with various company chemists and product specialists and have found what I think is the best solution for applying to the crack on the outside of the differential case: Permatex Spray Sealant Leak Repair (p/n 82099). You can check it out here: www.permatex.com/products/adhesives-seal...sealant-leak-repair/

I was very impressed by the support I had from a Permatex 'Senior Product Support Specialist'. The only problem is that it is totally unavailable in Canada. I can buy it on eBay and have sent, but it would end up costing more than $50 with shipping and take 3-4 weeks to get here. However, I have a sister in California and hopefully she can post a can to me today.

For the interior crack, I am talking to the chief chemist at Glyptal (thank you, Old Magnet!). He thinks they have products that will adhere to and seal the inside crack, over the old epoxy and the cast iron. But he asked me to send him the SDS for the synthetic gear oil that I run in Hanna. I expect to hear back from him tomorrow.

WoooHooo! In the wind ... the wind rushing through your hair... in your ears 😳... @ 8.2 KPH going down hill with a strong tail wind 😂😂😂
I happy for ya...

Please change the differential carrier bolts while you have it open. I friend had one break off and go between the bull gear and the bottom of the case. It produced results close to the damage you have. He grabbed a used housing and changed it out.
Thanks
ken

Thanks for the advice, Naylorbros. Will certainly inspect carefully. What your friend experienced would not be fun! The crack in my diff was, by all appearances, caused by the stud bolt--I'm guessing the bolt hole (no doubt a technical term I am missing here...) was not carefully cleaned, leaving some oil in it and, when the stud was put it, the torque cracked the cast steel. At least the crack is clearly associated with the stud.

Is that housing cast steel or cast iron?

According to those who are supposed to know, it is cast steel.
 

I doubt it would be cast steel. Cast steel would not crack like that and it would be easier to weld.

I would clean it well on the inside heat local spot up to some 200F to make sure there no oil in the crack then repaint some Glyptal on the inside of the casting as that is exactly what it was made for and you will see on the inside of your pony motor as well as many other gear cases. stuff is very rank so it an outside job. but seal from the inside as the outside will hold oil on it all the time. just clean well then clean it some more. It's all about the prep work when working with any kinda coating JB, epoxy, Glyptal, they all need good clean oil-free and a good tooth to their job well.

Any doubt on cast iron or steel......run a grinder spark test.