Non mandrel bent exhaust?

We are in the process of building our 318 fun motor. The set up will be a .030 over bore with KB 167s, 302 casting heads with 1.88 1.60 valves, cleaned up and gasket matched to Hooker comp headers and an Edelbrock RPM Air Gap with a Holley Street Avenger 670. We are strongly considering the Comp XE274 cam but have not fully decided yet. The rear end is an 8 1/4 with 3.90s and the trans is a 904 which will be rebuilt and a transgo kit installed. We are waiting on the torque converter selection until after the motor is finished and the cam selected.
I like the sound of the Spintech mufflers that I have heard and am considering them. My question is....Instead of running a 2 1/2" mandrel exhaust can I run a 3" non mandrel bent exhaust and get close to the same backpressure. I don't want to lose the last little bit of a bottom end the little teen might have by using too big of an exhaust but I do want her to flow well and I do want the sound... What do you guys think? 3" non mandrel is much cheaper and more readily available. Does the non mandrel neck it down enough to keep it close to a mandrel 2 1/2? Thanks