B-Body HP Manifolds
It's a known fact that B-body HP manifolds will work with the big blocks, with a few possible modifications. Tom Swope, from Accurate LTD, was kind enough to send along the casting numbers, as well as the changes that may be needed....

Left side for B or RB, The steering shaft sleeve must be cut back to use these.  Also if used with a 4-speed, the z-bar ball/stud at the bell housing may need to be  moved rearward to angle the z-bar for exhaust pipe clearance. 
    # 2843992, #2951865

Right side for B or RB, For RB use, an angle cut of 5 degrees at the head port end of the manifold to tilt the flange end towards the oil pan will add extra fender well clearance. The heat riser should be removed. 
    # 2806900, # 2899879 

Pictured below are the Hp's on a 383 '68 Dart, and farther down, on a '67 440/4-speed Dart GT

In these three pics, you can see the general placement of the driver side manifold. Being up out of the way, they will work with power steering and brakes. Note there was even room for the homemade steering shaft extension for the manual box. It does come close to things at the rear, but pretty much fits in like it was made for it. The exception here is the outer sleeve of the steering column.
You can see it was shortened up to the firewall for clearance, which means that these manifolds will NOT work with a column shift.

On the passenger side, theres plenty of room all around. Plug access is typical a-body....#6 got an access hole drilled in the fender to make easier. I will say that the exhaust on this car was completely home-made using standard bends from the parts store. No special bends were needed to get the pipes down from the manifolds.

Now on the 440, things are a whole lot tighter. They work, but clearance is at a bare minimum as you can see. Heat could be a problem in this installation. Also beware that these cars are all very different from one another and where one has 1/4" of clearance, the next might have interference. When things are this tight, theres no way to say if it will work on all of them without some work.

Although the passenger side manifold gets alot closer to the fenderwell, it's also moved upwards, which helps to maintain a small amount of clearance.

Pictured Below is the high-flow driver side manifold made by Accurate LTD. This manifold replaces the the OEM bigblock A-body unit and flows much better than the squeezed OEM casting.

Accurate Manifold