Look on eBay. Stainless steel exhaust pipe, reducers and bends are available and cheap to buy.
The reducer is likely to save you the aggravation of welding.
On a general note, avoid using U clamps on the exhaust. They will indent the pipes making it extremely difficult to dismantle at a future date, particularly if someone has overtightened the clamp. Mikalor make stainless clamps and these are a significant improvement over U clamps.
