24's would work but i think make their cycle rate higher than you'd like for reliability, but it would work all day long. You really just have a basic(no offense)GT40 setup. I'm running 30's with a Cobra Maf cal'd for 24's and it makes my eyes burn(non tuned). 30's would be right where you'd need to be with "room to grow" IMO, 42's are not necessary at all and will hamper performance within its allowable parameters.
24 lb'ers will be more than enough for your combo.
There is no real need for 30's or 42's.
Like Matt said the stock ECU can not really handle the 42's to have good driveability and good WOT.
Your car with just basic bolt ons will be right on the money with the 24's.
2014 V6 Deep Impact Blue Mustang, Borla Type S, 2.73 (Uughh need 3.73) 1989 Sonic Blue Mustang LX Coupe, 349 ci, 392 rwhp 424 rwtq....Mustang Dyno "There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." -Will Rogers-