3" or 2.5" Pipes?

I have a 1971 Duster the specs are below. I already bought a pair of Dynomax ultra-flow welded mufflers, which are 2.5" opening. I am planing on buying Doug Headers which are a 3" collector. Should I go with 2.5" pipes or 3"? I am also planning on doing an H pipe.

My Specs:
340 Bored over 30
904 Automatic
3.91 Gears sure grip
Martin Saine 3500 Stall 9.5" Torque Converter
Air Gap Intake
X Heads
Doug Headers
Holley 750 Double Pump with Porform Main Body and Metering Blocks.
Cam:
Lobe Center: 112
Advertised Duration Intake/Ex: 269 / 282
Duration @ .050 Intake/Ex: 223/234
Lobe Lift Intake/Ex .320/.329
Gross Valve Lift Intake/Ex .480/.494

Thanks!

Andrew