Any length of straight pipe will do.... 2" diameter... or 2 1/4"
No problems with noise, MOT's etc etc.
Doubt you'll notice any performance difference... but still a worthwhile mod... for perfomance removing the large centre box, and just running one small silencer makes more of difference.
If you get stuck, if your ever up this way again... I could get hold of some pipe for you... or even weld it on for you.....
Were the seats ok by the way??
Ian