port size and IIRC the bolt pattern differs from all the other SBF intakes. Note the Edel-B link below: like all suppliers, they note the intake won't fit the 255. Must use only (and it's the only one available) the 2-bbl 255 intake and pan.
http://www.edelbrock.com/automotive_new/misc/tech_center/install/5000/5021.pdf
Been so long I don't remember the chamber's look and pics seen to be impossible to find.