nobody knows how much the 5.0 GT will weigh yet.
Being as the 5.0 is still in the testing and development stages. It's still too early at this point. Also bear in mind, that Ford will not officially confirm any info, until after the 2010 Mustang has gone into production, this upcoming spring.
Looking at the current weight of the Z06/ ZR1/ and standard corvette i don't know if this is possible .