I installed that conversion kit in my sister's 77 240D over 6 months ago, and I have been very happy with it.
The kit includes four new style pencil glow plugs with a larger body that will fit in the larger threaded holes for the old loop plugs. No adapters needed.

If your 80 already has the control unit for the pencil type plugs, you might just need to get four of the special large bodied pencil plugs and throw them in. It seems like Mercedes used several dozen types of glow plug controllers on the w123 cars, so you might be safer just getting the entire kit and installing it. I'm not sure which type of glow plug control system your 1980 car has. I know the retrofit kit's control relay will afterglow in cold weather, which is a very nice feature.

I think we paid about $100 for the whole kit vs. $150 for a new control relay for the 77's loop plugs (it was fried). Best money we've spent on that car.

I probably have some pictures of them online somewhere, I just can't find them right now.

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1971 280 SEL
1981 Volvo 242Tic | 1980 Bertone Coupe