Geez dude, keep your shorts on!
Short answer- Yes, you can use your '72 K frame on your '73.
Long answer- You can swap the K frames, but if you have a sway bar on the '73 you won't be able to use that with the 72 k frame. Sway bars have to stay matched to the year range, ie, 67-72 or 73-76. That also means that the LCA's should stay matched with their K frame if there's a sway bar, because the sway bar mounts are in different places for 67-72 than 73-76.
No sway bars? No problems. Just remember that if you ever add one it will have to be the 67-72 style.