acceptable bs on a 18x8 mustang wheel...66 cuda

I have 4 wheel disc brakes going on my 66 barracuda and need to upgrade to 15" or larger wheels. I plan on getting leaf springs moved inwards for more clearance. I see alot of threads on wheels on early a's but it seems bs is anywhere from 4.5 to 6. what is an acceptable range that will work front and back? I have lbp, and 8 3/4 out of 70 cuda in car now. And yes i did search, just havent found exactly what im looking for. found some nice ferd wheels 5x114 backspacing ranges from 5.6 to 6.2...any help is appreciated.