I measured the distance between the eye of each motor mount while the towers were still on the old frame. Then ground a little off of the inner top edge of the frame to be able to get the right distance between the towers. And then notched the towers so the top edge that originally slid inside the frame rail could sit on top of the frame rail. Once I was happy with my measurements I just welded them in place. At this point in my build I was tired of drilling holes. haha
I also used the crossmember that bolts to the towers to help verify that my spacing was correct because I could see the marks where the bolts had been