I had the EXACT same "runaway oil pressure" problem last season with a freshened smallbock and new Melling HV pump. First time in all these years that this has happened to me. I replaced it with another HV pump (this time it was a Speed Pro but anyways) and problem solved.

BTW, the bad pump did not show any particular problem when I dissassembled it after removal (and I had opened it up/inspected it prior to original install). It had to be the relif valve but I could not prove it to myself so in went he new pump.

I feel for ya on the job ahead .... I think in hindsight, pulling the motor back out would have been easier than dropping the pan in the car