See **LINK**for an excellent set of photos.
OP appears to already have the lathe on raising blocks since he says "Lathe is on a Myford cabinet raised on bed raisers.". These are necessary to ensure bottom of mill column clears the tray.
The existing holes on the rear of the bed are for the taper turning attachment, having determined where along the bed the column needs to be so as to end up wih the mill column midway between chuck and tailstock, make a drilling jig plate whch incorporates the horizontal pitch of the existing holes and align the four holes of the mill bracket as required using thread taping size drill.
Bolt dril plate to bed and pilot the four holes, tap as required.
Just a suggestion.