Downpipe shortening may not be needed if...
If you can loosen your exhaust system clamps, particularly the one at the bottom of the downpipe, you may be able to "tease" it back enough to avoid cutting/welding.
I used visegrip "C" clamps and a pair of visegrip "chain" clamps.
I loosened the down pipe bracket till it was good and sloppy...
Then using the above clamps/grips, I would hook one edge of the C-Clamp and lock it on the clamp mount then around the pipe, then use the next one to do the same thing but lock a bit tighter... . I basically ended up "walking" the downpipe/exhaust back a full 1 1/2" and hooked up the 14cm.
It helps if you have a spare set of hands and a buddy working on the upper end near the coupling. He can call out directions... . back more, up/down etc.
I managed on my own but it was alot of crawl under, clamp... crawl out and look, crawl under etc etc.
It worked good.
In fact I recently put my 16cm back on and all I did was mount the 16 in place on the manifold, then loosen the downpipe clamp, let it pop back in place and suck 'er up with the V-Band clamp.
Try that before cutting anything.
Bob.