1.5 might not be enough but 3 might be too much. You can always spacer to go lower in rear but can't go back up. I did Hotchkis 2" front and 1.5" rear on my 69 and had to adjust the front back up 1" and the rear further down 3/4" to get it where I wanted it to be (fender lip just below top of tire). Every car will be different. Adjustable coil overs would be nice if you've got the spare change. Then again once its set you shouldn't have to adjust it again. Either way, be prepared to make adjustments. Good luck.
One thing to keep in mind is how the exhaust is routed over the rear axle. If your exhaust was installed prior to lowering the car you might have clearance issues if you go to low. I went with the 1.5 drops on my car and over big bumps I can hear the axle tubes hit the exhaust on the pass side. I also have a moser 12 bolt which has 3 inch axle tubes, so that could part of it as well. Its on the list of things to adjust lol.
I have 255x40x17 BFG Gsport Comp2 tires on my rear and the top of the tire is about even with the fender lip. The overall diameter of the tire is 25inches in the rear. You may have to drop down to a 35 series to get it even or close with the fender. I think a 18 inch 40 series tire is 26.7 high which would probably move the tire farther up into the fender. I don't know what brand of tire you have, so I would go over to the Tire rack. They list the specs for each tire and it includes overall height.
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