Automatic trans? 4l60E or 4l80E?
I don't know about the 80E, but the 60E uses a different bellhousing between the old SBC & the LS based V8's.
The bolt pattern is the same, however the depth is different. As is the flexplate and you'll need a LS type torque converter... I think there are adapter kits out there.
My swap is reverse, I used a newer LSx type 4L60E behind my 350 SBC and had a custom torque converter made for that swap.