The first metal/rap crossover was RUN DMC covering WALK THIS WAY togheter with the original creators Aerosmith.

Anthrax came years later with the i am the man single (this was before the release of their fourth full lenght album, on their first three albums they never used rap, they endorsed stephe; king novel story’s and comic books like Judge Dredd)

And, as far as i know ... it was not so very succesful,, because a lot of hardcore anthrax fans did not really like the rap influences, they were becoming a gimmick of their own success.

Talking about metal and rap crossovers ... because of the hardcore (genre) scene in the NY bronx, there were some hardcore / metal crossover bands (these crossovers made famous by DRI -Dirty Rotten Imbeciles- on their crossover album) playing around with rap in the mid ninetees ... biohazard was one of the first. Theres also a very nice official soundtrack, with various rap and metal collaborations released in those days ... it was from the movie judgement night

Goof luck with your stories, create them a little longer, with better info