I would try it with a known GOOD battery instead of the jump pac. If it turned the engine over with a battery you should not be able to stop it with your hands.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy