Modifying Fat Man Crossmember

[Handfull 54]

I was told by the people at FatMan that I could use a 79-90 Mustang R&P by elongation the holes on the passenger side. I was told just to elongate the inboard side. Has anyone done this and what type of grinder would you reccommend? I was thinking of one of those grinders used to port cylinder heads. My power rack is from a 74-78 Mustang II. I have a big block in my 54 pick up and it steers like a go kart. Iv'e only got 500 miles on it and I did have the pressure reduced in the steering pump but it is still too quick for me. Also has anyone used a manual rack with a big block? My crossmember was bought back in the late eighties and drilled only for the 16" on center rack. Thanks in advance for any help.