Lowering my 72 longbed

[BigA]

I have 2 1/2 inch droped spindels and 4 inch lower rear springs that I will install. Now on my oe springs in the back they set at about 10 inches and the new drop springs are the same hight. The question I have is will the new drop springs drop the 2 inches I need or should I go with a 6 inch drop spring. If everything is OK I can not wait to get started. I also plan on getting american tork thrust rims and I'm not sure what backsace to get. I have american soft 8 rims with 3 3/4 back space.