Step vans came with 292s so a mount should not be a problem if you can find one. I would suggest looking at a 70s GM truck parts book; you may luck out and a 292 could use the same mounts only on different set of holes in the frame. The 700r4 will bolt up to a 292 or 250. The driveline will likely need to be cut. Make sure the flexplate you use has the correct holes to mount the converter. If you really plan on hauling with the truck and going to an OD trans, I would go back to the 4:11s.
Don't get too carried away with a cam. As you want torque, you do not need a bunch of duration and you likely will not be turning more than 5000 rpm so pick the cam accordingly. You need to get some expert advise on the head; that is where the big money comes in. The valves and flow rate need to match the cam and depending on who you talk or listen to, you may not even need to cut the "dividers" out of the head or add lumps. The carb choice is yours, but the more there are the harder it is make the linkages work and tune.
I would keep the compression low enough to run 87 octane. You are going to push a lot of air and needing to go to 90 or 93 will cost a whole bunch more.
Just opinions..........
