magnum 360 into a 74 duster that had a slant 6

I used this for my 5.9 magnum: ATP Z332 Flexplate | RockAuto

1994 and older used the weighted flex plate, 1995 and newer had the weight on the converter. Usually. After all, we're talking about Mopar here, so almost nothing is 100% certain.
I won't "disagree", because as you say; never say never... That said, in my limited experience with 5.9 Magnums I have found the opposite:'95 or '96 and newer (I've had a '97, '99 and 2000- OBD2 era) have had weighted plates and neutral convertors; but have been told the early OBD1s had a non-weighted plate and weighted convertor (no personal experience with early Mags)... so I guess the moral of the story is: ALWAYS check what you actually have- assume nothing! Just use the right convertor to match your flexplate, and vice-versa.
PS: 5.2 Magnums *ALL years* use non-weighted flexplates and neutral convertors- just to keep everyone on their toes.