Hi Raymond and welcome!
Do a search on English wheel and you will find pages of people building them.
Rectangle tubing is the strongest frame material. The deeper you go the weaker your frame. A bolt on lower arm seems to be the best bet, it makes it easier to align the wheels. Plus if you want a lower adjuster you can make one and bolt it on.
3" dia lower anvils seem to be the normal. 2"or 3" wide will be fine. 8" dia upper is a good size
One more thing. If you are trying to find out something, and have did the search just ask!