There are too many variables there to be able to predict that with any measure of accuracy. You'd be time and money ahead (money saved from having to rework anything) by bolting up the 9" as it is now and then measure some things first.
With an 8" wheel, you're going to have to measure the distance between the wheel mounting flange on the axle and the leaf spring. Take about an inch from that measurement and that'll get you in the ballpark on wheel backspacing. You'll also need to measure the distance between the wheel mounting flange on the axle and the fender lip. You could also just measure the distance between the leaf spring and fender lip. 1/2" less than this measurement (leaving 1/4" gap on each side of the mounted tire) will give you an outer limits measurement of what your tire's section with might be.
All of the above I did with the original rearend mounted in the car and was pleased that my tire and wheels actually fit the first time even though my differential was about 1/4" off-center.
I was able to fit an 8" wheel on the back of my 67, using the OEM 10-bolt, by using 5" of backspacing and trimmed the fender lip a tad. A 235/60-15 or 245/60-15 tire fit my car just fine, but I measured everything first to be sure and I really, really, really recommend you do the same and not take someone else's word for it. All cars are going to be slightly different and not always in a good way.
If you wanted to avoid some calculations, just have the guys who build your rearend housing make it match the OEM 10-bolt measurements.