Also, Ford essentially obsoleted the keystone gaskets you pictured. For better cooling in the keystone engines they recommended '46-8 round hole gaskets and minor redrilling on the center row of holes in early heads. Info is covered in the '37-48 engine manual, I summarized the simple work in an older post:
Got it...Ford V8 overhaul manual, '37--48:
Look at central vertical row: Topmost hole gets drilled to 3/4", skip one, third from top goes to 5/8", next two nada. All done!
Late gaskets are the same R and L but have a front and back due to some small hole size differences...so because of this, one goes crimp up, the other crimp down!
