Have anybody used these Monroe shocks before for sagging leafs?

The springs don't care which rear end is mounted to them. Replace them now, your future 8 1/4, shock plates and u bolts will fit just fine when you get them. Trying to raise your rear with overload shocks (or air shocks) just puts excessive stress on your upper shock mounts, and can ultimately cause issues there- they're not intended nor designed to bear the weight of the car. They also usually require beating the seam of your fuel tank flat for clearance of their oversize diameter.
Just get the springs- you know you need them anyways.