Driveshaft contact with th400 ??

Trying to figure out the driveshaft on a 71 Chevelle 454 12bolt TH400. It is a project that didn't come with a driveshaft. I have 3 driveshafts with a Th400 Yolk on them. My question in how much contact with the tailshaft splines can I get away with? It seems any of the yolks will only go in past the seal about 2 1/2inches at most. My one diveshaft has a short yolk that seems to be the best candidate. I push the yolk all the way in raise the rear to put it in the rearend saddle and pull it back an inch to seat it in. By the time I do this I am only getting 1 and 13/16 of contact with the splines (car is hanging by the frame on jackstands). This measurement comes from 1 and 1/4 of the yolk inside the seal and 9/16 of the shaft that sticks out past the seal inside the yolk. Can I do better?