Unfortunately, I don't have the statistics since 1976. I can give you some numbers, if you'll just give me a moment.
As of August 2009, there were 4,311 federal offenders who had committed first- or second-degree murder. Of these, 457 have more than one murder conviction. While I can't answer your question about the length of time they spend in custody, I can tell you that as of August 2009 only 26% of multiple murderers had been paroled, compared with over 40% of single murderers.
Concerning the precise questions you've asked, I will undertake to get that information for you and bring it back to the committee.