A body K member

Pencil it out. It's mainly a matter of cost. Conversion mounts for your existing K-member and you save the labor changing it, buy and convert to the '67 idler and swap the K-member, or buy a '68-72 V-8 K-member or the 73-6 V-8 K-member and stock motor mounts and swap those in. For street use, any of those are decent options. For performance use, The last two or aftermarket K-member would be better. The stock K-members need rewelding and gussetting to fix sloppy factory welds and to reduce flex. The level of performance can escalate the cost significantly.