The Illustration portion of the master parts catalog also shows an oil slinger - basic # 6310. But the parts list for the 351M & 400 doesn't show one, as you can see from the snippet below where things are listed numerically.
My guess is that both the shop manual and the illustrations are generic, meaning they were prepared for any 351M or 400, but that by the 80's that part had been eliminated. So I don't think your engine had one.
And you cannot put one in w/o removing the timing cover. So if you have checked the timing chain for slack and it is fine I'd do w/o the slinger. But you don't want to get that close to the timing chain and not check it.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI