18 x 8 wheel backspacing

[71streetbird]

Hi guys
I'm building a 71 firebird factory subframe with a complete umi 2.5 suspension kit, on an 18 x 8 wheel what backspacing would be good for lowering the front possibly below the lip of the fenders I have coilovers

[79T/Aman]

to get that low, performance is out the window so you may as well use drop spindles and with only 8" wide wheels you can do it, back space 4.75-5"