You will never be able to drive a sub frame car at a 5" rocker height, that subframe would be at 3". You need about 5" to the subframe which will set your rocker at 6 1/2" to 7". At that height the fenders and quarters will barely cup the top of the tires. you will have to build a full chassis and channel the body for a "ride height" of 5".
It is difficult to make a 67 / 68 look low. The wheels wells being rounded on the 67 / 68 make give the illusion of the car being taller then it really is. A 69 will look lower then a 67 even with a slightly heigher overall stance because fo the squared off fender openings on the 69.
Our 69 Firebird looks lower then our 67 Camaro, even though the Camaro is nearly one inch lower. Below are 2 of our cars. Now I specify "ride height" becuase a lot of cars can sit low on the show field or even the fair grounds at idle. By ride height, I mean driving down the road, hitting bumps and pot holes ride height.
Project Prodigy, the 67 Camaro, is probably one of the lowest "ride height" F Bodies around with a 5 1/2" to 6" rocker height at "ride height". It has a channeled body, full frame, nothing hanges under the car at all, the rockers are equal to the lowest point on the car.
Project Prodigy is 47" to the top of the windsheild frame which works out to a 4 1/2" to 5" drop.
Project EmptyNest, the 69 Trans Am, is probably one of the lowest "Ride height" 69s around with a with a 6 1/2" rocker height. This is a Speedtech Subframe car and has a number of tricks done to the car so it can drive so low. The subframe is installed with no body bushings which is equal to a 1" body drop, also, it has zoomy style collectors on the headers so the headers are only a 1/2" lower then the subframe, which puts the header right at 5" off the ground. Our customer inner fenders among other things allow the front wheels to steer at that height.
Project Empty Nest is 47 3/4" to the top of the windsheild frame which works out to a 4" drop. Project EmptyNest is 3/4" taller the Project Prodigy.