The pipe angle looks good. Plenty of room to get an O2 sensor in there. O2 bungs are also cheap and easy to weld in. I would choose the position you want the sensor, mark it an give the area a bit of a tap with a hammer and remove enough material to get the bung t fit in snuggly. I also found that once I had the general shape of the hole, get your trusty Dremel out and try to get the hole as round as possible. Don't forget your hammer if the O2 bung rocks about a bit!
It really is a pity about the welding. Stick welding is a bit of an art. Some tips from when I was doing a fair amount is the heat the rods up in the oven. If you have a torch, heat the area up that you want to weld. It worked well for me.
Keep it up.
