So if i sorted a Set of 3.5'' alloy Piping and made my tip to suit and nice 25mm boss and 6mm n75 welded in should neaten the install up..

I could always Use the ''Brazing'' way off welding alloy and actually make it my self...No?
 
3.5" pipe will be a little too big, look for 85mm and I suppose you could braze it..

The other option would be too buy 3.5" tube and then reduce the ID with something..
 
hmm it wouldnt be the best fit, 034 seem to use 85mm ID on there pipes.. maybe the rs4 is actually 85mm ID.. If thats the case you will need to find some 3.5" pipe with a 2mm wall thickness
 
As I've said you really need to find out what size MAF is needed.

If the code is written for OEM 70mm MAF and 550's and you use 630's and 83mm MAF i think your scaling will be out.

On my ibiza, which is the same setup as Alex used on his Ibiza the code is written for a 70mm MAF but uses an S4/74mm MAF with 630cc injectors and 3 bar MAP.