What was it originally off? If it is just an old type fender mounted mounted starter relay then it is unlikely it would be rated for 200amps.
The problem with using a relay that does not have a high enough current rating is that, at some point, the contacts may weld themselves together. If that happens you would likely lose all eight glowplugs.
Check parts store listings for a glowplug relay for a 1986 F350 6.9 and it would fit and be rated high enough.
I did a search on the topic of using a fender mounted style ford starter relay/solenoid for glow plug duty and I read back through about 5 years of posts. It seems like people are split about 50/50 on this. You can read over and over that it wont work, then there the guys who have been running that setup for years and recommend it to everyone. On other engines its common to use a starter relay and I know the 6.2 crowd does it.
Some guys say the GP circuit pulls 50 amps, others say 100, other say 200!
Some people say that the relay must be rated for continuous use but that really doesn't make sens either bc we only use our glow plugs for 5-10 seconds at a time.
I read that these relays are rated to 100 amps. I guess I will run it and let you know what happens. I will keep a pair of cutters on the dash in case I need to quickly kill that circuit in the event that the relay sticks.
I just wanted to give an update. My starter relay is still running my Glow plugs just fine. The truck is a daily driver and is started with glow plug assistance about 4 times a day on average. I will let yall know if the relay burns up. So far, so good.
I wanted to follow up on this again for anyone who is considering using a fender mount style starter solenoid in place of their glow plug relay. I have been running mine for over 15 months without issue. I regularly run my glowplugs for 10 seconds and have not had any issues yet.