I can tell you that your info isn't correct, at least from a math stand point. 1.5" is greater than 25mm. 25.4mm is exactly one inch.

Also, don't forget that in 1978, LRT used two different wheel sizes for the front and rear. In 1979, all four were 15x8.

I had the factory wheels for a LONG time, but now I cannot remember the specifics of them. Age takes it tolls...

But in the end, you have nothing but room in the rear fenders on Dodge trucks. I even ran 15x10 Cragar SS's with N50's and while they filled out the space well, they were hardly jammed in there!