I know in the SRW world of SuperDuty the 250's get a 2" block and the 350's a 4" block. My dually has what looks to be a 2" one also. Assuming this has to do with being a DRW truck.

 

I had to convert my F350 from 4" to 2" because of the 5th wheel I tow.

There is a Ford SB concerning this but I don't remember the number.

 

I know when they listed tailgate heights the 250 and 350 DRW were similar where the SRW 350's were about 2" higher. So it must be that the SRW's just get the 4" block. The springs are heavier I believe than the SRW, at least according to some spec sheets from the Ford Fleet truck body builders specifications.

 

Some CC trucks (450+ IIRC) won't even come with blocks, there's an optional "low deflection" package on them that adds 2" spacers.

 

there is also a difference between 4x2 and 4x4 pickups.

My 4x2 F250 has no block.