The 68 and 69 B body manifolds will fit good with the low deck in a 67 and up A body. You'll need to cut the steering column housing about even with the firewall and do away with the column shifter mechanism and convert to floor shift. The C body HP manifold also will fit. That's the one that sometimes requires milling so that the outlet is closer to the engine. The passenger side B body HP manifold pretty much just falls in. With the RB engine like a 440, People have also made those fit, but it starts getting pretty dang tight.
