Yep - they can be a bit out there.
There's plenty of online E/T calculators out there - I was posting this one with reference to the other driveline calcs provided.
What numbers did you plug in anyway - they are "garbage in - garbage out" calculators.
Remember its rear wheel HP and vehicle weight in lbs.
Stock XE 351 does 1/4 in 16.3sec and 0-100 in 8.9sec........a real slug!!
Numbers on ford website for 351 XE are 149kW@4300rpm - see this link.
The weight listed is for the lighter 6 pack. Your 351 XE (I am assuming you have a 351??) with you in it will weight about 1650kg (3600lbs).
SO......re-enter 3600lbs and 170RWHP (149kW=200HP. Minus ~15% for driveline loss = 170HP @ back wheels). What did I get........16sec ET @ 85mph. These calculators do give you reasonable ballpark figures - only if you enter correct data.
PS: A 3600lb car will need about 450HP for an 11.5sec 1/4 mile.