Normally one uses the hole at the back of the block by the distributor. That is the only one I have ever used.
The later blocks could be drilled for oil pressure there though. It looks like someone took out the sending unit an put in a pipe plug in the top photo and I would check that pipe plug to make sure it was tight if I didn't use that spot for the sending unit.
I checked Autozones part location thingie and
http://www.autozone.com/autozone/rep...essure+Switch#
They do show two pronged oil pressure switches like the one in your photo for a lot of 79 and up engines though.