I think much of the needed work for Tru64 would be simply to add *at least one* 64-bit platform for Parrot's "core" platforms.
Preferably an LP64 one, instead of an LLP64, since LP64 would be more likely to shake out bad assumptions. But if LLP64 is more easily available, so be it. Superplusgood would be to have 64-bit both ways, that is, LE and BE. *) E.g. http://www.unix.org/version2/whatsnew/lp64_wp.html