The exhaust port size and placement, and the flange bolt pattern is AFAIK unique to GMC (228, 236, 248, 256, 270, 302) and definitely different from Chevrolet stovebolt (which is similar in appearance but smaller). I suspect the exhaust port size (but not the position) differs between big port and small port GMC heads. AFAIK a 270 may have either.
If the primary opening in the backside of the flange is the right size (not smaller, slightly larger OK), a "foreign" header (but probably still from a 7 port engine with individual #1 & 6 exhausts and siamese 2-3 and 4-5 exhausts) can sometimes be adapted (lots of work) by splitting the flange (in the center, or between every port) to move the primary positions closer or apart. Engines with similar bore pitches are probably the best candidates.
