They made a 0.63 OD T5 for both the Camaro/Firebird and Mustang. This mated to a 4.57 rear gives a 2.88 final which is too shallow for most engines in trucks to handle on anything but flat road and no wind. The 0.68 OD gives a 3.10 final which is probably still iffy without short tires. Keeping the engine in the middle to top of it's torque range at the desired speed keeps both it and you happy.
