The problem becomes clearance. The muffler reliefs in the floorpan are just in front of the axle- to run the tips out from standard mufflers from there would require a 180* bend before heading out the side. The alternative has always been to move the mufflers forward, but then they're hanging pretty low to clear the floorpan, and can take a beating. Another alternative is a slimline muffler (think old school glasspacks).
For great clearance on a rowdy side exit, maybe consider Boom Tubes...
Dr. Gas! BoomTube Mufflers and Boomtube Tips (drgas.com)
