I would stay away from blocks. they WILL cause axle wrap, and can make the ride just as stiff as an add-a-leaf. They can also break, and shoot out from between the axle and spring (bad) . They also create a weird twising force on the rear axle, by moving the axle away from the spring.
I hate it when people do what I just did...but, I fixed it for you. Sorry...but, nothing good ever comes from using lift blocks!
Wheel the truck stock, save your money...then send it all to Marlin for a SAS kit when you can wheel the snot out of it.