There are no commercially available headers for an RB or B motor in an earlky A. There is so little room the best you can do for a SMALL BLOCK is 1 5/8 tubes, if you want to stay inside the frame rails.
First off, with an RB motor, the exhaust ports are right up next to the shock tower on the passenger side. So that has to be addressed. Your best option is to go low deck with a set of stock iron heads, or any head with the stock port location that isn't any wider. It doesn't hurt to go higher, as you will have to build these yourself, or have them built. The shock tower will probably need to be modified for pipe clearance, which means moving the shock mount out a touch. The next problem will be trying to fit all those tubes somewhere. It may take a full fenderwell set, or a two inside the frame, two wrapping the frame to make it work with the little space available.
Probably the best option is to go with a Mustang II front end. Not cheap (but about as cheap as a front conversion gets), but you will have a lot of room for the motor, and gain a lot of room for exhaust.
If you try and build a set of fender well headers, the best way is to keep all four tubes against the wheel well to save the turning radius. There isn't much room in there for a four square arrangement like you usually see.