The main reason to do the Shelby drop is to change the camber curve on your car when cornering.  Lowering the UCA mounts makes the UCA sit with the ball joint higher than the pivot bushings, so when that side of the car is being compressed, the arm is already in the part of its radius where it is moving inward as it is moving upward.  Stock has it moving outward while moving upward.

Typical Shelby UCA relocation gets you 3/4" lower.  Moving the spring perch outward will increase the effective spring rate and sometimes raise the car slightly.  Cutting one full coil is usually about a 2" change in ride height.

All of that stuff varies with the engine, tranny, your body weight and the accessories on your car like AC, headers, etc. Aftermarket UCA can also make a huge difference in ride height.

I had great success with the Progressive Motorsports front coils and the Shelby drop.  They put the car right where it needed to be.  Then, I changed to an aluminum engine block and the car got higher, so I cut one coil.  That put the car 1" too low, so now I have purchased another set of progressives and will cut 1/2 coil to hopefully get back to the perfect ride height.

The real reason to go out as far as possible on the control arm is to get more spring travel per inch of wheel bump travel. This allows the lowest possible spring rate to be used. Lower spring rates mean lower forces at the pivot points of the suspension which in race applications allow lighter components which mean less unsprung weight.....and so on. In our restomod world it is still a great thing to have the lowest possible spring rate to get the best ride. I know most are worried with the "perfect" stance and cut coils and such. That is why my car has coilovers at all four. I love low spring rates! Most say my car is "bouncy", but they only see the tail lights on the track!

I can't speak with direct experience but I think you are headed in the wrong direction by cutting a stock spring to get to your ride height after the perch relocation.  As shown in previous posts, the advantage of the perch relocation is to use softer springs to reduces stresses, etc.  Cutting a stock spring will increase the spring stiffness and more than likely counteract the advantage you seek with the perch reloaction.  If you are going to use a stock spring, you might be better off not moving the perch.  There are a lot of posts on this site on this subject and if you do some google searches on perch relocation you will see a lot of info on the use of a softer spring to acheive the ride height.  Some have gotten away with using stock springs but it appears that in some cases these were older if not orignal springs that had probably lost a little stiffness over time.

Also, I would suggest that the best way to get to the ride height you want is perhaps a third of a coil at a time.  If you cut a full coil, not only will you have a stifer spring that you may not like but if it is then too short, you can't add it back.   Yes, it is more work to take it apart three times but you'll get there without overshooting.

But, before you start cutting your stock spring, look into the 6 cylinder springs that others have suggested.  You are doing a lot of work which you will be undoing using the wrong springs.

Did you decide on your combination of springs/drop  etc??