Classic/Antique Car Repair: 196? Ford Mustang, old ford mustang, ford mustang

Question

The car Hi I have an old ford mustang with an inline six in it. The woman I bought it from told me that is was completely original with nothing done to it. It has 90,000 miles on it and the vin number is 5F07T756918 I know that certain things can be known about it, just by looking at the vin. How do I know if it is original? And does this car have any worth to it if it isn't completely original? Would it benefit me to put in an aftermarket small block 289 in it? I am also trying to figuer out exactly what year it is. I thank you for your response in this matter.

Answer
Try the following web sites:
http://www.autoinsurancetips.com/mustang_vin.htm
http://www.musclecardrive.com/vin-decoders/ford-mustang-vin-decoder.php
As for replacing the engine, my experience has been that old stock with numbers marching what is under the hood is the most valuable.
Brad
Nice looking car.