In general catalytic converters can have the exhaust flow through them in either direction. If this weren't the case there would be a visible marking on the converter that would indicate which direction the exhaust should travel through it.
If it is a direct fit converter for your 1991 Nissan 240SX like the # MF22767 which is a straight pipe with the same flanges on either side that is confirmed to fit your vehicle it would be reversible. view full answer...
The SeaSucker Monkey Bars Roof Rack # 298-SX6000B needs to be mounted with the dual cups in the front and the single cups in the rear. view full answer...