drop my '68

[GMC Canuck]

I keep looking at my truck and keep thinking it needs to be about 3" lower. The truck has it's original stock springs so they're probably a little sagged already. I have about 6" clearance front and back wheel well to top of tire. There is about the same clearance from axle to frame at the rear. How far down can I go before needing a c-notch? What drop do I need to keep it level? I was looking at coils front and rear and maybe blocks on the rear as well to keep it level. Besides coils, shocks and adjustable panhard bar is there any other surprises I should be aware of?