It is my experience that a stock iron wide base four barrel manifold will do a very fine job for a small street engine if the plenum is ported. My engine is 301CID and made +325Hp with a ported Iron manifold and all the other stuff. I also never ran any faster with the Blue Thunder than the Iron 4BBL manifold and I am a very early adaptor. It is my opinion that a 320" or larger Y with highly reworked heads, big valves, headers etc would be necessary to utilize all the potential of the BT.According to conversations with John Mummert and a careful look at a BT next to the porting job I did on my Iron 4BBL the major thing the BT has is larger runners and wider radii in places where the passages under the carb enter the cross passages in addition to the modified plenum discribed below.
I ported the plenum under the carb based on ideas from David Vizzard's book "Performance with Economy" which is an interesting and useful source. The key is to remove the sharp edges where the 4 holes enter the passages. These holes are drilled during production of the iron manifold and provide no transition for flow downward from the carb to horizontal flow in the passages. By radiusing these areas of the manifold a big improvement in flow profile is obtained. This is a lot easier to do when the web between the fore and aft holes is removed which also increases volume under the carb, a good thing. Once you do all this you will see a severe restriction where the fore and aft passage dives down to the lower cross passages. This restriction needs to be opened up in the floor of the passage which has the corderoy surface. If you can reach it there is a ridge in the top that can be smoothed as well. I do not remove the center partition between the fore and aft runners because this has the effect of making the manifold a single plain type. After all this I also fill the area between the holes and outside of the carb flange with a high temp 2 part hardening plastic.
I have run with a 4 hole 1/2" (stock) spacer and a 1" open spacer. The 1/2" 4 hole idles better and is a little smoother running but the 1" open spacer seems (seat of the pants) to make more power. I have a 1" 4 hole for my truck which also has a ported Iron4BBL but I can't say how that runs at the moment.