Wouldn't a self pardon effectively be an admission of guilt?
Yes, and he would have to specify which crimes he had committed lol.
Ford's pardon of Nixon wasn't that specific. It was very broad:
They still could have nailed him for sabotaging the peace talks in 1968.
No one knew about that until after he died (2007 IIRC). Nixon probably went to his grave terrified that the Chennault Affair would come out, as it was, as you say, not within the timeframe specified in the pardon.
LBJ didn't want to blow the country up with its disclosure. I suspect he also wasn't sure that it wouldn't backfire and elect Nixon with a bigger margin than expected. (Humphrey was surging when Nixon pulled this.)
Today, no one cares about blowing up the country, and lots of folks look forward to it.