The Z28 y pipe is bigger on the converter end than 2". IIRC it's 2.5". The 40471 y pipe is 2" until they merge together into a bigger pipe before the converter. Coming out of the converter is also bigger than 2" I think it's 2.5 also. The y pipe that goes to the 2 "mufflers" splits into 2 2" pipes. The part number that I am talking about is 42566. Where did you find part number 42537? Can you post a link please? I can also tell you for sure that the factory pipe from the engine to the cat on my nova is 2.25. Coming out of the cat is also 2.25 that runs to the muffler. I think when they went to the intergrated head the exhaust side was improved. The camshaft has more lift and duration and the exhaust pipe is bigger. Look at pipes munber 45426 and 45537. They are both 2". The 75 to 79 is 2.25 inch. I know what you mean by a tight budget, I think that the 292 manifold is the way to go in this case. I also agree that there might be enough movement in the exhaust system to bolt up to the 292 manifold when the time comes. I am now thinking of a possible Z28 type system for my Nova thanks to you. Do some research here I think that a member here got the best power out of a 305 by using a short piece of 2" pipe than stepping up to 2.5". Look at posts by Deuce Coupe to start. You could save $$ by buying the Z28 parts starting at the y pipe in front of the mufflers, hang it yourself then have the shop hook it up to what you have left of the old head pipe and make a new head pipe up when the head swap is done later. Just an Idea. Jay 6155
