67 slant6 3ontree conversion to 3byknee A230 driveshaft problem

I believe I had a 903 3 speed tranny in my 67 Barracuda (not a A745).Whatever is stock is what I had.My mechanic had no problems with the above switch until he said the driveshaft was too long by about an inch and a half,because the slip yoke was not sliding into the A230 deep enough.To make a long story short,he is having a specialty drivetrain shop modify the yoke,replace U joints,and rebalance the shaft.Is anybody familiar with this switch?I thought it was straight forward,but I guess I was mistaken.Any input would be greatly appreciated.