Installing a C-notch without the axle in.

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I'm installing a McGaughy's 4.5/6 lowering kit on the chassis that will eventually end up under my '87 GMC. The issue I'm running into is how to figure out where the notch needs to go so it's centered over the axle. All the rear suspension is removed from my frame, so I can't just measure center that easily. Are there any measure points I can use to figure out where I need to make my cuts and bolt this in? Thank you! -Marc.