69 Firebird 18" backspacing

[steve g]

What's the correct backspacing for a stock first gen? I'm looking at the Coys C5 wheels in 18x8 and 18x9.5 with 235/40 front and 275/40 on the rear. The backspacing available looks like 4.5 for the front and 5.5 for the rear. I've got speed tech control arms and 2" lowered hotchkiss front springs. Stock rear, although I'll probably go with a shock crossmember to move the shocks inboard.