i had this same problum with the same speakers in my 91gt...i ended up selling the speakers and i bought some nice 6x9's and put them in boxes in the hatch area and left the factory speaker location empty. i used some speaker wire and extended the factory wire to reach, i just ran the wire out the side of the grills. you cant see the wire with the rear seats up
when i asked about this, i asked what speakers will fit in the rear and no one had a forsure answer so i just went with the 6x9's in boxes....seems odd to me that so meny people replace there factory speakers but yet no one has had this problum or not meny have?
i also thought of cutting that beam, but i figure its there for a reason..most likely structural strength which would really come in handy in a roll over and i wasnt willing to sacrifice my own safety just to get some speakers to fit
P.S. whats a good speaker to use for the doors?