1998 GMC Jimmy engine aint turning over

Engine Mechanical problem
1998 GMC Jimmy 6 cyl Four Wheel Drive Automatic

i recently installed a new (used) 4.3 in my gmc jimmy and it dont turn over so I take the starter out n check it and it works fine so I hook it up to the starter wires while it aint installed in the motor n it works fine but when installed it dont turn the motor over I was wondering if I possible installed the flywheel backwards would this be possible