On my totally-stock 74 318, I installed TTI's step headers AND their 3" H-pipe duals. These goodies replaced my previous 2" duals w/o x-over, and 318 logs. Carb at the time was a cruddy TQ from an emissions 440 that ran pig-rich, and I can tell you there was NO loss in bottom end with the new way-oversized (by conventional wisdom) pipes. I think the benefits of the headers vs. mani's will far outweigh any potential loss due to oversized pipes. If I was building the 273, I'd either run a big fat single exhaust with a Flowmaster Y-pipe, OR duals using some long cone reducers from the headers down to like a 2.25" tubes and an X-pipe.
If you know how many HP you're working with, you can hit the Dynomax catalogs and find mufflers that provide unrestricted flow up to that number for singles or duals. Very helpful info.
