So, based on the dyno sheet that looks like mine, with JUST a 20% loss from the drive line, you would have these numbers vs. my stock dyno sheet...

264.9 * 1.25 = 331.125 HP ------------vs. stock----------- 249 * 1.25 = 311.25 HP
294.7 * 1.25 = 368.375 torque --------vs. stock---------- 252 * 1.25 = 315 torque

And that's not even counting the low numbers for the Mustang dyno.

If you factor in the extra 12% for the Mustang dyno loss/low numbers you get

264.9 * 1.470588 = 389.5588 HP ------------vs. stock----------- 249 * 1.470588 = 366.1765 HP
294.7 * 1.470588 = 433.3824 torque --------vs. stock---------- 252 * 1.470588 = 370.5882 torque

So it looks like you aren't getting that bad of numbers after all.