Hello,
I have been looking into the exact same size oil arch for over a year now, my friends and i, we are really good fab guys could build one if all dimensions and slopes and clearences are known, but getting those for each size arch are very difficult. The big guys will not give up the blueprints which is understandable. You could convert a wood arch, but they are NOT efficient with oil, and that is my main concern. With materials and labor, its almost worth it to buy a leader arch and be done with it because you know it will work. I got a price on one from leader about a month ago, and for $3944(2x8) it seems to be the way to go. Best of luck