I will trade you problems  I have duals that ran into glass bottles then into a set of mufflers and dump under the mid of the underside floor.  it is pretty loud and there is no room to make a nice affordable set-up that exits from the back.

as for what you are asking maybe a bit more info might help.  what motor? what you have running now? and probably any muffler would fit in the stock place for you.

Rather than just throwing a cherry bomb on there, I'd recommend a header. The money will be better spent, as you get better performance, not just less back pressure. If the current system is just old, and it's time to replace it, do a header with a flowmaster type can, matched to a diameter compatible with what your motor needs to breath.

While not a 4 cylinder Pinto, I have a single, 3" Flowmaster 50 ("suppose" to be a quieter 3 chamber muffler) on a 318 Valiant.  The DRONE comes on at 1,900 RPM and stays until 2,600 RPM. Right in the normal driving range.  It is so bad (No, not bad, HORRENDOUS) that it truly alters your senses. It is like a sonic warfare device. I vote a big NO for any chambered muffler.

I would like to go to a 2.25 exhaust and have a ranger header I plan on using.  I also got a Walker quieflow muffler as I don't want it to be real loud.