I have a one owner car sence 1973.
The car has a 198 slant 6 with the snake skin landau top.
I did some research and supposedly the car wasn't suppose to have the snake skin top. That was reserved for the 318 V8 and the 225 skant 6.
When it comes to what was allowed in terms of vehicle equipment, some requirements and prohibitions were based on actual reasons—some equipment wouldn't fit with (or work without) certain other equipment, that kind of thing.
Other constraints were arbitrary: some product planner decided the [whatever decor package] was only available with [whatever engine]. Those ones were a lot more flexible; if you got a dealer who wouldn't just reflexively parrot the printed line about what was/wasn't available, they'd go ahead and order you what you specified. Worst the factory could do was reject the order, but Chrysler needed every order they could get around the time your car was built; making money was more important than "Oh no, there's a car out there with a snakeskin top, and it only has the 198!". The engine under the hood made no difference to whether the snakeskin top could be installed.
All that said: where did you read that the snakeskin top was available only with the 225 or 318 engine? I'm not seeing any such restriction in the original or revised '73 Duster/Valiant brochures. It is possible you might have got bad info on this point; the internet is full of it.