I'm thinking that it would take approximatly 8 inches of drop on the shackle to get the 4 inches of lift in the blocks. You only get half the lift of any extended shackle. You would have to put blocks on both ends of the springs and I'm sure you would become the laughing stock of the board. Blocks are not ideal, but unless you are having axle wrap issues I don't really see the problem with them. I doubt you are getting much more wrap, in any, than the Marlin rear springs have.
Get a u-bolt flip kit and relocated the shock mounts higher on the axle housing.
