Been there, done that with 67-72 slant sux K frames. If you make a jig, make it to accept the shouldered K frame bolts. I built it off a V8 K frame. One in the jig is a 67 slant sux K I modified to V8. Will be converting idler bracket to 68 up which is why the 67 only one is missing in the pix. See pix below.
67-72 LH mount is the same from slant sux to V8. I cut the slanty sux RH mount off, built up a RH V8 mount and welded it on, as well as modifying the LH mount to V8 specs. I imagine a 73-76 jig could be made off a V8 one in similar fashion. Hope this gives you some helpful ideas.
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