You'd pretty much start with a local radiometric survey company (there are a few in Perth) using a 50 litre doped sodium iodide cyrstal pack (a bit heavy for a drone) and scintillation counters in a crop duster flying at 40 m ground clearence dead centre down the road at 70 m/sec for the full 1,400 km road length.

Post process the 256 value gamma spectrum, correcting for cosmic, aircraft signature, remove the mean average W.Australia backround signature, run a full rolling NASVD to peak sharpen and look for 32 and 662 kev twin peaks.

Then you'd start with the local area ground search.

Given the lapsed time already there's a good chance the slug in question was either magpie'd on site or has been picked up in a tire and gone off route.