if it was fine before it can only be something you have done make sure the washer is there and use the correct torque setting. ft may not be tight enough
strange ,one to keep you up all night... gearbox somehow fowling flywheel.. wrong size flywheel... does the engine turn free by hand when put together and no internal scrapes/noises? you removed the crank out of the engine case? any end float? are you using a flywheel locking tool? is the rear of the engine case free from things poking out...? Is it the same clutch? grasping at straws... ;D
