Its one of the reasons alot of our faster streetcars used to run their mufflers all the way back there,some even split their exhaust into 4 pipes and 4 mufflers partialy due to the available space.
Regarding mufflers outside the framerails there is a limitation there on most of our cars since some genious who engineered these cars put the leafsprings there aswell,but if your leafs are moved in under the framerails it might just work or if you are running a fourlink or ladderbar suspension.