67 slant6 3ontree conversion to 3byknee A230 driveshaft problem

So all is well,the original slip yoke from the 903 was cut down 1.5 inches,new u joints installed,and the shaft balanced.My mechanic said there is still 3 inches of contact overlap between the output shaft of the A230 and the modified slip yoke.I have bought a 1970 on 903 slip yoke from a FABO member to compare it with this modified slip yoke to see if I can figure out what happened (a member here said an older slip yoke could be longer,which would seem to me to mean the original 903 tranny I had in the car would have to have more overlap with that slip yoke).Thanks for all your help.