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I have done a little research on the subject, and of course found all kinds of answers and ways to do what, but the general rule i read was do not end with a vertical edge. THe reason i am looking into this is due to the new found crack in the frame. it's vertical and about 8in forward of the steering box. I did a big write up on it in my build log:
I think you may be confusing what you want/need to do and how to do it. First looking at your pictures, the PO REALLY made a mess of that frame! I honestly would not recommend repairing that frame, but replacing it. It's been so cut out and welded on it's unlikely it's straight any more, and definitely won't be after trying to repair it. trying to fix that by boxing is like trying to fix a compound leg fracture with band aids. A 250-350 frame should not be difficult to find or expensive, everyone wants the F100 frame. Next best choice would be to find a left front frame rail, cut the rail at the cab mount area and graft on the new rail. Once you decide what way you want to go, PM or email me and I'll walk you thru the process.