BGC?
TM, the manifold as sold new, had the plenum, fuel rails, and brackets to fasten them to the manifold. It also included the hardware. Aftermkt plenums won't fit. As I recall, sold for about $600.
I would determine the value of the piece by deciding how fast you want to go.
IMO, if you stay w/ the stock block and heads, then I'd stay w/ the modified stock intake. A stage engine can benefit more from the BGC.
My production combo went well into the 10's, dragging w/ it, a goodly number of tuning issues. The manifold and heads were Ruggles ported stockers, and a production short block.
Lastly, you will most likely have to "scrounge" a manifold, if you decide on the BGC. Don't think there are any being produced, and if they are, you best go back and check the archives on dealing w/ those guys, before you hand out a CC# ...