Do you have any salvages nearby? Any wiring harness for a DS-II ignition system can be made to work, although for a perfect fit it needs to be for a 460. However, all the DS-II harnesses had the same wiring, it is just that the sending units, like for oil pressure and temp, were in different places, as was the distributor. So the length of the wires may be wrong.
Plus, if you have a factory tach a harness from a 300 six won't have the ground that tells the tach to go into 8 cylinder mode. But that can easily be added.
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