If it is a B20E then it should have what is called today a 'Free flow exhaust', and came with a 60 mm system from the factory, which is n/l/a. I looked for one last year and never located one. Both Bosal & Starla used to make one, (which included a front resonator muffler as well as a larger resonator in the tailpipe) and all the parts are still listed on the Classic Volvo Parts diagrams, but not avail.
If you put anything like the stock B20B 50mm (2") system on there, which is probably what it has had, it won't run as well as it should. It is best to at least get the larger front header pipe, and splice in a 60 mm size system.
Several years ago what I did was took a 240T system and used the whole system up to where the front muffler was, put a straight pipe to meet the header pipe, much like the IPD system. You can find these from Starla/Walker made in Sweden.