My intake manifold was manufactured by Kelly Coffield. He and I were in the same boat: 9.5" tall block and C302B (Brodix BF300 in Kelly's case) heads but no IR manifold for our combination. Lucky for me, Kelly decided to cast his own manifold. He did one run but last time I spoke with him, he wasn't planning to do any more.
That EFI Hardware manifold looks great but do they really have them on the shelf? Also, don't make a cast-in MAP plenum your primary criteria. There's room in a 351C to hide a plenum below an IR manifold such as Hall's.