I would recommend using a newer glow plug relay from a 1980 on model for your conversion.
The thread below has some info on wiring the 4 prong plug on the newer relays.

Fast Glow for my 76 240 D

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76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.