Driveshaft shortening question

Small Block said

fastback is on it. Do it that way. Do you understand what he said?
Small Block

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Glad I found this cause " No, I don't". I'll be going 7.25 to 8.75 and figured to shorten driveshaft the difference between axle center to u-joint center of the two. Are you doing the same 1BD71 ?