What does your title say? Your title should tell you whether it's a GT or LX. You can also run your vin # at:
http://www.mustangworks.com/articles...n/351swap.html and see what it says. I don't know about the cutting. Perhaps you have to cut a little to make a 5 speed work in lieu of an automatic. It may have originally been an automatic. If your car is a GT it should have an 8.8 rearend. The 7.5 was used in LX's until 85. From 86 and up all Mustangs were equipted with 8.8's.