There are a few points to consider with in this case twin rear shocks and they can get confused.
First the length of the shock, this will be the distance between the two eye bolt position on your H-D shock and this will be measured generally when uncompressed, so off the bike is the best way to measure this or with the weight off the rear wheel and off the ground.
Then there is the length of travel, this is the distance the shock can move when fully compressed to un-compressed. (generally you have to take what the manufacture says for this)
Now we come to pre-load, adjustment and sag, so basically what this is about is getting the shock absorber in the right position when it has a normal load, (i.e. the rider and or what ever else on the bike normally) so it can move both up and down the ideal amount to absorb the bumps and irregularities, so if set wrongly and it is too far either way it will loose the ability to absorb the bumps etc. For example if it is set say fully compressed it will have no movement to to compress further and be in effect solid suspension.
We will not go into damping here.
This is why getting the shock absorbers set correctly can make a very big difference.
Not all shock absorbers are the same, you can have the same overall length but have a different amount of travel, i.e. say 3 inches of travel as opposed to 4 inches, the more travel you have the more the shock can move and ability to absorb shocks to some extent, so may well be more tolerant to not being set quite right regards preload and sag.
Street Glides and Tourers have the same frames etc, with the Street Glide running the Shorter rear shocks, it is generally consider they will ride better with the longer standard touring shock length, longer shocks will affect the steering angle, so to some extent change the handling characteristics and steering. I believe the longer shocks generally also have more travel.
So a longer shock with more travel may well be able to be set up to the same overall seat height when seated on the bike as the shorter shock with less travel, though it may however give a taller seat height when unladen, but should give a better overall ride.
Using lowering blocks is not ideal as this changes the position and angle of the shocks, thus the forces and leverage and in some I believe rare cases can lead to failure of the swinging arm mounts.