That is a good point, and something I hadn't considered. General consensus I get is headers don't do anything for an engine that wasn't designed with performance in mind, except change the exhaust note. (With obvious exceptions being manifolds that were a bottleneck in the first place.)
Yeah, don't get too hung-up on the "flow/cross-sectional area" on this, it is the headers ability to isolate the pressure-waves and subsequent rarefactions of each cylinder
that make them effective. While it's true if You're going to putt around in Your 273 and never see the high side of 5K a set of 1&1/2" OD tubes will be a great torque boost,
a set of 1&5/8" tubes aren't going to "kill" that effect by any means, but idle-2500 may be a tad less...You do have less ci./cyl. than a 225 Slanty after all!!
As far as headers and flow goes, Ya might want to watch this.................
