Here is a photo of the tach drive on the back of my 401 and the keyway. (I always assumed it was square)
Here are part numbers and lengths of the different cables/shafts used; from a GMC masters parts book; I was surprised to see how long they can be, but they are for the Big truck series.
Here are two links; same vendor on EBay. Looks like he can make you one and he has different lengths/styles;
square drive or
keyed-tang or
boxed.
http://www.ebay.com/itm/370916872713...84.m1423.l2648
http://www.alkydigger.net/proddetail...rod=ADTACHCBL-