Are all front spindles on a 1966 mustang straight 6 the same size?

I ordered a replacement spindle for my car since my original was all stripped....when I received the spindle, it was thicker. My disk brakes do not fit this spindle. The reason for my question is the guy I purchased it from insists he took this spindle off of a 4-lug six cylinder 1966 stang. If that were the case, wouldnt it match my current spindles?? Did they come in different sizes or could this spindle be off of a 5-lug V8 since it looks thicker and more heavy duty. Is there any way to tell??