I am going to throw this out there for a friendly debate.
I will be putting a mild 360 in my 87 D150 that will have long tube headers.I have all of the mufflers and pipe already to run either a 3" single exhaust or a dual 2.5 setup.Make an argument for one or the other if you like.
Motor is 9-1 .030 over 360 with a smallish(torque)cam,aluminum intake,4bbl in the 600-650 range likely,and some light porting on the heads.
Stock tranny/converter and will have no more than a 3.55 gear.

What would you run and why?

I'd do whichever one packages easier in your application. Either will have sufficient flow capability for the sort of motor you are building.

-Jon

I think flow capability will be similar between the 2 systems. Two into one may act like an H or X pipe with better scavenging and more torque... also quieter. If you prefer the traditional sound of dual exhaust on a V8 that favors keeping them separate.

(1) 2&1/2 duals. (2) traditionally how it is done/looks/sounds better

I am leaning towards the 3" single because I,
1)Like to be different

2)Truck is already single exhaust and will appear "stock"

3)I already have a 3" mandrel bent tailpipe and stainless long case 3" in/out muffler from a departed project.

Though the twin Magnaflow mufflers and resonators I have lying here are creaming what about me?!Maybe they will find their way to the 75 Newport.