253's can sound good with a twin, but they can also sound like a sick chook and cackle a lot. It all depends upon where the crossover is, pipe size etc. Singles have a deeper sound and avoid the chook noises. Also depends upon the cam and heads. One example of how the same exhaust can sound different with a different engine: my Overlander (red 308 TH400) was reasonably quiet with a 2.5" single (factory manifolds). I changed the engine to a bog standard VP 304 with maxitork manifold and carby off the red engine. Pipes were adjusted to match the EFI exhaust manifolds. The damn thing sounded like a V8 supercar, was so loud it'd give you a headache. Point is, every engine is different and what sounds good on a 308 may sound like cr@p on a 253. That's why I suggested you go and see an exhaust shop with an old fella that used to build exhausts for these.