You just drop the top half. Or xor it? That'd make for some nice traffic-pattern art ;)

Now the reason this doesn't work is that the address length is not encoded, so if you make the address longer, no one knows about it and they [routers and destinations] just assume the wrong address and send it on or discard the frame

Kids these days can fit 64-bits into a 3-bit hole, and if you squint hard enough it still sounds like a robot. Amazing times, I tell you.