Unless it’s got built in power feeds as some mills have, the others including your Myford are a simple collection of parts. I wouldn’t think twice about pulling it to pieces. While I don’t know about your exact mill, I suspect you want to remove the hand wheels, feed screws and then the gibs. Then slide the table out onto something close to the correct height. I’d also bag each set of hand wheels, dials and any screws and mark each bag for which end of the table if came off of, and for the Y axis. Time spent doing so is repaid later with no questions about where each part goes. Removing the knee is a bit more complex, so if possible I’d leave it attached.
As far as re-indicating it back in after the reassembly. Again that’s just a bit of technique, some time, a plunge type dial indicator, or better imo, a .0005″ or .0001″ lever type dial test indicator. Anyone that understands how to use the mill or replace the spindle bearings should have no issues replicating the accuracy your grandfather did. And that part removal is an excellent time to solvent clean those feed nuts, feed screw threads and the way surfaces. You might be surprised at how much smoother everything operates after that.