I would also suggest that while the battery is out, to clean ALL the corrosion where the cables run under the battery, and give a good eyeball to where the cables run towards the front of the battery under the weldiment. the cut on that part of the battery box is sharp enough to eventually wear a cut through the power cables. just replace any cable that looks like the insulation has been sitting in the corrosion or is cracked from rubbing.
Imnsho EMI in forklifts also is just about as likely to be from induced current, static discharge and/or counter EMF as it is from 'ground loop' (feedback).