never use "pipe", it is inconsistent in its wall thickness....for any part that could compromise your safety as well as that of others...
plus 1/2" is way too small for the forces that the radius arms go through...
I am sure you could get with Greg and mod some stock radius arms....most get 2" square tubing with a 1/4" wall to use with the stock arms.....
do a search, Plum Crazy has some nice ones he made in his build thread....
l8r, John