Lowering rear end

[old51sedan]

Ready to lower rear of truck by installing 4" drop springs. I know this had been talked about a lot, I also did several searches on it but can not find an answer to my questions. Have a 69 C-10 Sb, SS. Truck already has a rear track arm, do I need to replace this with an adjustable one or can I still use it. Just curious, but if it needs to be adjustable has anyone ever cut one in half and welded in an adjustable sleeve like in a screen door adjuster, only bigger? Also I understand I need to install a tapered (4 degree?) shim on each side by the trailing arm clamps to keep the pumpkin/driveshaft straight to eliminate vibration due to it being off center. I only want to do this once, but right also. Thanks, any help appreciated.