Not sure if this would result in your symptoms, but I just had a 60a 5-speed installed behind my a12 in my ute - We needed to use a combination of the different clutch, flywheel and throw out bearing from the a14 and the a12. (Luckily I have an a14 ready to go in soon) The gap between them was too great for the clutch to disengage fully using the a12 flywheel/clutch combo, and then needed the shorter bearing carrier - maybe your clutch isn't engaging properly?
Especially since you said you can free-rev it easily??