Simply stated, the 73-87 hoses are a good bit longer.
They go to the frame rail in front of the crossmember.
The 71-72 hoses go to a tab on the crossmember, well back of where the 73+ hoses go to.
Also, the end is different...
71-72 on top
73-87 on bottom
Don't get shook up though, the 71-72 hoses are still plenty available, so use whichever your truck calls for.