Is there any special procedure to replace the 4 rear sparkplugs? any help would be appreciated.

Nothing special, might need a couple extensions including a wobble.

I believe your Northstar is the same basic setup as my 94, which means you should remove the coil pack assembly. It's bolted on with four 10mm bolts. Use a full 3/8" extension. Be careful not to drop the bolts. When bolting it back on I suggest screwing one or two bolts partially, which will allow you to lift the coil pack just enough to give you a little extra room to be able to hand thread the other bolts. I just replaced my plugs over the weekend. It's an easy job. I owned a Lincoln Mark VII LSC, which was a pain to replace the left side plugs because there is little room.