Stock Magnum EFI 1/4 Mile Times in an A body
105 mph should be much faster than 13.5. I'd expect a stock magnum would outrun my stock LA 360 and it went 13.9 with 2.45 gears/no headers.
Yeah, you could run a quicker time while trapping at 105mph but both the ET and the trap speed are pretty reliably calculated based on power, weight, typical drivetrain losses, good launch, decent atmospheric conditions, etc. Of course, it's a ballpark figure only.
One fun thing to do is to check published performance numbers on production cars. Punch in their curb weight, advertised (crank) horsepower and you'll almost always be within couple tenths and a couple mph. It's depressingly accurate. :D
Obviously, with customized hotrods, there are a lot more variables As mentioned above, the driver, track altitude, atmospheric conditions, track condition/prep and how good the owner is at tuning are all significant.
In any case, 13.9 is a very respectable time for a stock 360. What's the setup for your car running this 360? Do you have a limited slip?