Mine is 2.5" diameter cat back, if your going for a full system you might be able to run over the chassis instead of under it, would involve moving the cats or just having one. There's space to do it if do something with the cats.
I've only got one silencer which is still quite a big unit, any smaller and it'd be too loud for me. I found the rear silencer prone to damage off road which is why it got ditched.
If you want it quiet still consider keeping a small rear silencer.
People who've heard mine will confirm it's not loud, quite a deep tone to it and from cold if parked too close sets off the neighbours car alarm

even when floor it you don't get deafened