trust me, exhaust parts NEVER go together without a fight, i had the same gap as you, plus one pipe was too short and had to be extended, bare in mind i ordered the prochamber for longtubes as well... i'd bite the bullet and take the car down to an exhaust shop and have them do it
old post but no solution i am haveing exact same issue and i have same setup 1 3/4" long tubes to 3" collectors to 3" prochamber to 3" dumps. and my headers measure about 18 outside to outside edge and my prochamber measures aobut 17" outside to outside and it looks just like the pic posted above. all are correct part numbers got them brand new for my year and 5 speed
i have seen this happen before and what i have done is attach one leg and used a pry bar on the other to get it to fit has worked for me before just got to put some muscle into it though
vibrant red i just recieved my headers, prochamber and dumps from mac and they all look similar. mine are all the correct part numbers for my fox and the correct ones i ordered and look like the pics orig. poster posted. prob. gonna do what ghost stang said mount one side and use a pry bar and hope for the best or bring it to a prof. and cut and weld it.