First off: Welcome to the site! The 4L80 that would come behind a L31 350 (heavier vans, trucks, ect.) will have the bell housing you need. Along with the normal stuff... the torque converter, dipstick tube, ect. You will have to move the crossmember back/modify the mount, ect. You will need a different trans yoke/driveshaft, trans. cooler tubes & a controller to operate it. Some way to make the speedometer work also.
You may want to spend some time doing a Google search for: "4L80 swaps" the basics will be the same. Doesn't matter if its in a Chevelle, Camaro or a truck. Hope this Helps, Lorne