I dont know if you are interested or not, but this tool will do exactly what you are looking for.

http://www.hmpipe.com/Shape.htm

We use these on pipelines all the time. You already have a torch so all you would need is the jig. You will get a perfect cut each time and you can even set the jig to bevel the pipe so if your wanting to make a root pass you can.

The only other thing I would suggest is set all the post, cut holes and run the pipe through the post.