[Wisconsin_Snake]

The side pipes used on 91 where very restrictive. You probably want to update both with with newer less restrictive side pipes.
Someone here is starting up the Classic Roadsters again and are located in Canada.
Their ID is gn_n_60_shelby, probably want to contact them to see if they have side pipes available.
Otherwise there is a place that sells the muffler section (you'll need to search that out) and get someone to cut your mufflers out and weld the now ones in. Again with will be an upgrade cause they will be less restrictive.
I just hope yours are not chromed or with the nickle finish like mine.