In 2005, the only difference between an F250 and an F350 was the blocks Ford added between the top of the axle housing and the bottom of the rear leaf springs. The spring part numbers were exactly the same for the the 250s and the 350s. Buying a 350 just gave the owner more wheel travel and thus the ability to carry more weight. Is this true for the 2015 and 2016 models?

 

While the springs themselves are the same, the F350 will have an overload leaf spring on top. The F250 camper package will include the overloads, and a rear stabilizer bar. This is for at least the 2011 (my truck), and I am relatively certain correct for all later years.

 

I can't speak for the newer models but my 2006 F250 has the camper pkg, which includes the overload spring (see pic). My dad has a 2004 F350 SRW w/o camper pkg & while he does not have the overload spring, his spring-pack does has 1 more spring than mine.

Note: OP was correct on difference of Block sizing between 250/350

But.....If 05's Spring p/n's are the same,
I wonder if my dad's is different from mine due to 04' vs 06' model years (obviously Front is way different)

 

2012 too.