If you look at enough listings you will find the rear crankshaft seal for LA 360 and 5.9L Magnum engines do overlap, somewhat.
If an older part number seal is found listed for a LA 360 but NOT for a 5.9L Magnum, it is only because the older part number never got updated to include the newer magnum engines. A newer part number, such as FelPro BS40651, is found in listings for both older LA and newer Magnum engines. So they do basically take the same rear seal.
I just measured a brand new 5.9L Magnum 360 crankshaft where the rear seal rides. If I did it correctly, the measurement was 2.965" using a micrometer. I also measure an LA 360 aftermarket crankshaft where the seal rides and it was 2.9634" using the same micrometer. So that is only about .0016" difference. Also, the biggest difference is the LA crankshafts have the ridges ground in for the old style rope seal, and the Magnum crankshaft is ground completely smooth without ridges, so that could be the main difference in the two sizes.
Find out from whomever is asking you for this measurement WHY they need it, what exactly they are looking for. I am guessing they are making sure you have the correct main journal size, not rear crankshaft seal size. A 5.2L Magnum or LA 318 would take a different main journal size than a 5.9L Magnum or LA 360.
