Over axle pipe diameter

[cabriolet]

Hi,
I have a 1986 XJ-SC and have had a stainless steel exhaust fitted by Longlife. The fitter had difficulty in getting clearance for the over axle pipes and I thought it was fixed. As it has been good weather naturally I have had the top down and in doing this I can hear a tapping when going over uneven ground. This sounds like it is from the exhaust, and under inspection on my ramps I see there is very little clearance between the suspension spring and axle. I need to take it back to the fitter and I need to query the size of pipe used. Does anyone know the maximum size you can get away with as the original is just under 2” and the replacement is about 2.5”. I have read somewhere that you cannot go over 2.25”