I have a 95GTX with the 657x and my understanding is the rotating parts are the same among 657 however the upper rod bearings, cyls, rv cover/carbs are the offshoot parts. If you are able to find a crank it may be exact replacement or you may need pistons/pins/bearings to use with your existing motor parts. The 657x has larger cooling jacketing and is pursued by some for the heavy duty concept. Otherwise a 657 could be used as a replacement with a slight change where the pulse line is in the block, you will not see this until you try shaping the line there.