I think my old '73 Power wagon used dual 2.5" offset inlet, center out turbo style mufflers mounted side-by-side with the inlet on the low side and center up higher before going over the rear axle. It was not really loud, but you could still hear it.

My '86 W100 with a 318, I think used 2" flare at manifolds that up sized to 2.25" right after the manifold connection, then used (I think the brand was IMCO?) chambered muffler and I think it was a 3" in, 2.25" out, but we mounted it backwards with the 2.25", and a single 3" out, over axle and exiting under the bed side behind rear wheel at a 45 degree angle, end cut parallel (45 degree) to the bed side. Not sure if that hurt flow, but it made a near stock 318 sound pretty good.

There are 3" in, and 2.5" out mufflers that might work good reversed. It sure makes a simpler install with just the single 3" tailpipe.