I'm not %100 sure but I've been told by a few good sources that on the front 16 x 7 with 225-40R16. Rear, 16x 8 with 245-45R16. I am in the process of doing the same to my 69, I'm not sure of the offset yet.
From what I've come across that sounds about right. In the 1999 August issue of Chevy High Performance there is an article about a tricked out 69. The wheels and tires used are 17x8.5 and 245/40 in front and 18x10 and 275/40 in the rear. There is a good photo of the car with the tires on and looks good. There is no mention in the article about any modifications to the stock dimentions of either the wheel wells or axle housing. In fact, the article states that a 9" was fabricated to the stock dimentions.
On my 69 Camaro RS I'm using 235/60/R15's in the front with no rubbing. On the rear is 255/60/R15's (though I have seen 275/60/R15's used without rubbing but they are 28" tall) I'm using a rear spacing of 3.75". Hope this helps.