Slant six spark plugs and numbers

The peanut plug head started in 1975. These are the ones that take a 5/8" socket to change.
But any slant 6 head will bolt on to any slant 6 block. If using an 81+ head you will need an 81+ valve cover though. All '60-'80 directly swap among those years.
Your car is a 1973. It's 2021. Something has probably been swapped over all those years since it was new. No big deal.
I have a 1974 block that I'm putting a 1977 head onto, and then my whole works is headed for an 85 pickup.