I didn't read all the posts but the oil pressure reminds me of when I was running an engine built by a machine shop whom had forgotten to install two vital, internal oil plugs...
There are 3 different 360 oil filters, your year came with the medium length one, I think it was a little longer then what I run on my 360, the NAPA 1068 which is a Wix but I forgot what the Wix number is (something with 5 digits I believe).