Never used that kit, but I solved the rheostat failure by removing the rheostat and wiring the two posts together. Easy fix. Lots of light now. Just pop the dash out. I usually just reach up behnd and shove it out. Of course the correct way is to use a hook tool to pull it out from the front. Either way it comes out easily.
Some have found you can get even more light by cleaning the instrument cluster carefully. Gets dark with age.
The 123 used an oil pressure line up til 79 I believe. Then went to an electic sending unit. Might be off by a year there. Have an 81 123 parts car with electic sending unit and a 78 with a plastic oil line to the dash.