2 inch pipe, that's 2 3/8" O.D. Schedule 40, will bend if you put enough tension on the wire. Go atleast 2 1/2" on your pipe, and I'm talking 2 1/2" inside diameter or 2 7/8" outside diameter. Personally, I'd use 3 inch pipe. The 1 1/2" you have will be just fine for cross bracing. You can either saddle it as previously explained or else egg it with a sledge hammer and weld it up. If you saddle it, it will look much better.
If you use the template as described, the saddles on each end will have to match. What I do is lay two pipes of the same size side by side. Then I take a file and run across them both such that I get a scribed centerline on the ends. Then I take my pipe templates and align my marks with the scribes.
Pipe sizes all go by inside diameter up until you get to 14 inch pipe or so. Hence, if you go to the hardware store and ask for 1 inch pipe it will be 1 3/8" outside diameter. If some layman tells you that he has 3 inch pipe and its 2 7/8" outside diameter, you are buying 2 1/2 inch.
Drill stem works great and it has work hardened. It does not tend to bend the way new schedule 40 bends. The problem is that the work makes it become magnetic and difficult to weld. You can resolve that by heating it with a torch and allowing it to cool but that also affects the temper. If you saddle it, you have already heated the cross brace part.
4 feet deep holes are good in clay. In limestone, 2 feet is plenty if you are in the stone and concrete the pipe to the stone (you aren't budging the stone when you pull the wire).
I made the mistake of using huge cedar posts on my first fence. In 1990, a grass fire that started two miles away took out that fence. I'll never use wood again on a perimeter fence. I spent hours bursting through limestone to put in those wood posts. Now I use 3 inch pipe or square tube and I'm done for the rest of my life, unless someone runs off of the road and takes it out.