(new guy) suspension/coil question

[telly444]

hey guys, I just bought a 66 shortbox and I'm trying to figure out what the previous owner has done to the suspension. I know the front is dropped and the rear has 3 inch blocks under the axle but I would like to lower the rear a bit more if I can. I can't figure out if my rear springs are lowering springs or not. they are about 13 inches with no load on them. with the truck on the ground I have 5 inches of play from the axle to the frame. what would you guys say the least amount of distance I could have from axle to frame without a C notch before I get into trouble? are these lowering spring? what would y'all recommend I do? leave it be or lower a bit more in the rear?