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I have just started doing the 4L80E cable drive speedometer conversions that I've added to my T5 and T56, and 4L60E ones already I do.
You send me your tail housing and I modify it for cable drive and send it back to you with either a gear adapter for full spline output shaft, or a gear you install on an o-ring style output shaft. On the o-ring style you must use a non vented driveshaft yoke.
If you have any questions please pm me for details.