I have used an extra long oil pan bolt (1/4-20) with a nut on top of the flange where the pan attaches. This bolt secures a clamp I fabricated that captures the down pipe and secures it to theengine via the 1/4-20 bolt with the nut. That fixture keeps the pipe from working against the manifold flange and breaking the gasket. Without this, when the engine rocks in the frame it stresses the pipe because it's secured to the frame, hopefully with fabric exhaust pipe mounts. That said, the factory mount for the tail pipe on my truck is rigid.
I believe cars used the donut type gasket while trucks used a flat gasket sandwiched between a flat manifold flange and a flat pipe flange. At least that's the way al the truck engines I have are configured.Either way it's important that the pipe not work against the manifold. There is a thick graphite gasket you can get that prevents leakage due to the extra fire from a truck engine. I have used this successfully for the last couple of years. You need to buy the Cadillac application for 2" pipe. I do not remember the vendor's name at the moment.
I believe there was a factory provision for securing the down pipe to the engine on some years. If you're going for factory originality perhaps one of the parts-book gurus can find a factory part that you can copy.
