If you look at the rods possition on the piston pins on the underside of the motor when its assembled the Off-Center shows up with the pins offset to the side of the available pin between the pin bosses. An on center motor like the Stage II and Series I, II, and III will have the pin in the middle of the pin when looked at from underneith. The intake has relocated passages accordingly to account for the different offset in the cylinder banks and the front cover will need to be milled down to make everything on the front of the motor work. The rods might be different too but I don't know since I've not delved that far into building one.
The On-Center blocks are slightly easier on parts due to the decreased piston side loading but thats a full on race motor detail. I think they also tend to be stronger more because they were cast later with more improvements rather than any fundemental structural improvement. But like I said, I'm no expert.
