Exhaust Pipe Diameter 273

well being a fabricator by trade,and having built multiple exhaust systems,id say 2.5 duals it a little overkill for a stock 273.considering the opening on the exhaust manifold is like 1 3/4"-1 7/8" it would be like hooking a fire hose to a residential hose bib.plus its heavier,harder to route,and more expensive.if you look at the commando 273 exhaust its a very well thought out system.the y pipe dumps into a single 2.5 mid pipe with a 2.25 tail pipe.the pics posted of the 2.25 system with magnapacs is also a good example of a well crafted system,but its the largest id go for what you are doing.jmo.