No,you don't. My 331 with every bolt on you can get idled fine with stock ecu.I don't know a whole lot, but I know from reading that you probably need a tune. I needed a tune because of an x pipe, and a 80 mm throttle body.
I think the reason you were told you needed a tune is because of miss matched parts. Like the 80mm tb. JMODo you have a check engine light on...Like I said I don't know ex what I read. I have a chip because my Mustang guy said I needed one. No cats,big Accufab,X pipe