Well... let me say i went to the same calculations for my car... The difference is that i went with 14 inch wheels but the final diameter of the wheel/tire combo should be the same...
Besides, i respected the actual original high of the car, measuring from the ground line (the floor) to the highest point in the roof... According to some data of a 351 Boss i got back then, that high should be 127 centimetres...
My suspension was modified in the front and back... Nothing scary but the last owner liked the 4x4 look on this mustang.. jeje!!
My problem was that y had to drop the car 1 3/4" in the back and 1 1/2" in the front and that meant my clearance was going to be much more complicated...
I decided to go all the way with the results of my calculations and ended up with no problems at all...
The tires i have are 225/75/14 all arround... The final diameter fits perfect for me, in the big wheel opening our cars have and the 225 step looks "normal"... The final Diameter I got is the same as that spec list of a 351 Boss gave.. Dont remember the actual measurements but i do remember my final diameter is 0.9 centimetres bigger.. (3/8" more or less)...
In the front, my cofiguration is the maximun this car car get, being a little lower than I was expected (in my opinion my car should be 3/8" higher in the front, from the coil springs...
In the back, something more wider wouldn´t hurt... the 225 step is Ok but it could go to 265 with no problems if you want a modern look... I choosed the same tires all arround because i´m going after a stock originality look with my 14" wheels and big round tires.
If you are going to use Magnum 15", look at Jorge´s green vert that has an excelent setup with the same parts...
This picture has a horrible resolution... Iphone 3G at night
... But shows a full side view of the car and that can show you the looks of my tire config...
This one is more like a front view but show you that in spite of being into de fender, the front wheel turns all the way without any problem what-so-ever
Well my friend, hope it helps a little!! Keep us posted!