Classic Stainless exhaust systems

I need a new system for my 94 lse but I need cats. Companies like SS do a full system in 304 stainless including manifolds and lambda sensors for an acceptable price but no cats.
Is there any reason I shouldn't just get cats welded into the Y piece as long as the bore dia is the same and they are physically no bigger than the original ones? It's going to be quite a lot cheaper than a full bespoke system.

I have a Double S system on my RRC.
Two different stainless steels in use, the rear silencer & tailpipe are as shiny as the day it was fitted - over 15 years ago - & the front silencer only has heat related discouration.
All the pipework incl tubular manifolds is surface rusted but no significant thinning of the metal on the tubes. So it's 'stain less' not 'rust free'.
I had the tubular manifolds off last year & the faces that mate with the gaskets were heavily corroded with deep pitting. Refaced them to get the worst out & used a small amount of firegum to ensure seal.
Exhaust clamps supplied & the metal joint olives have all been replaced as they rusted away so obviously mild steel.

Note that their warranty is non-transferable & requires you to have the vehicle inspected annually - something that surprisingly I can't find mentioned on their own website (although I haven't logged in to theirs so might have missed it):
"All Double 'S' exhaust systems carry a lifetime guarantee. Double ‘S' will replace or repair any part of their Stainless Steel Exhaust Systems found defective through corrosion during the period that the system remains the property of the original owner. Provided that the system is inspected by a garage or exhaust fitting centre annually to ensure that the rubber and other mountings are in good condition"
https://www.britpart.com/parts/perf...tainless-steel-components/defender/nrc8888ss/

That's useful to know as I've heard varying things about quality and fit. I think 304 ss corrodes more than 316 but it's cheaper.
Who else makes off the shelf kits? I'm not sure these are the same (disregard the presence or not of the 8" extension piece)
https://stainlesssteelexhausts.co.uk/range-rover-classic-4-2l-petrol-1992-1995-exhaust-system/
https://www.famousfour.co.uk/new_parts/ff_part?part=24942
https://rimmerbros.com/Item--i-RA1080F

SS themselves have given me a good price on theirs.
I've asked a local place about grafting a cat into a Y piece.

I have SS cat back systems on both my P38 diesels, there is surface rust around welds on the older system but nothing serious. The stainless exhaust on my MR2 Mk1 is over 20 years old and looks as good as new.