Or "Frank's Weldless" clamps that forum member scottly produced for a while when Frank stopped making them.
One nice thing about Frank's design is that one could use it NOW, then if later on you did a full frame up restore you could use a welded type then.
I have worked up some stuff to make my own but not actually cut my K3 frame yet.
The "keys" prevent the tension of the joint from only being handled by the bolts, but after much thinking driving to and from work and other idle cpu time I think that fit up may be really tough to weld in and have it fit nice. I was thinking about 1" round tube gussets to replce the stock bent flat steel too, but now thinking more like 3/4" flushed out to the outer tubes would be sufficient and nicer, even 1/2" may be sufficient really.
A simpler joint like this but longer like the other version is what I think I will use. There is a huge amount of "grab" in the flat surface using a 1/4" button head cap screw torqued properly so that in fact the tension IMHO is not only taken by the bolt.