has anybody used these frame rails?

When I bought my '68 the original builder had them installed from the rear crossmember forward. Through the floor (minimally) at the rear. It had the factory K member mounts and the factory upper control arm/shock mounts welded in. It held a complete factory k-member. From experience: note that the K will sit about 1" higher into the car (the factory rails are not 3") with a stock top of the rail from the floor measurement (required if your using the factory radiator support and inner fenders). I had to have 1" spacers fabbed to lower the K from the frame, leaving the rails at stock height (top of the rail) reduced the engine tunnel clearance.