Best bet is to do as DSeries4 did and get a Airresearch (Garret) replacement turbo from either an aftermarket or Agco dealer.  My local Agco dealer has gotten me my last two turbos for 426's and they were only around $450-500 each (a couple years ago) sourced from an aftermarket supplier they deal with and not an Agco part.  I had checked into changing to a different turbo and after researching as much as I could, any possible gain would be very minimal if you did gain (on the 426).  The turbo supplier you choose to use should also list a conversion kit for converting from the Ray Jay to the Airresearch, the kit just consists of a short drain tube, a couple fittings for the new oil hoses, and some hose to supply oil and drain the oil from the turbo.  If you follow the link, it is to a page for a turbo and conversion kit, I think they are well overpriced on the kit, but is to show a "nicer" conversion than the one I had that just used hoses instead of the metal lines.   http://www.spencerdiesel.com/Detail.asp?Part=SDTMW-%3EGAR%20190XT%20KIT" rel="nofollow - http://www.spencerdiesel.com/Detail.asp?Part=SDTMW-%3EGAR%20190XT%20KIT    Your series 3 should have the Airresearch turbo though and shouldn't need the conversion kit, here is the listing that place had for just the turbo.  Again I think the other supplier my local dealer used was cheaper, but it was a couple years ago and the prices may have went up since then.    http://www.spencerdiesel.com/Detail.asp?Part=SDT409040-RP10" rel="nofollow - http://www.spencerdiesel.com/Detail.asp?Part=SDT409040-RP10   The following link is to the A&I turbo which would be the same as the one above.  http://www.allpartsstore.com/ItemDetl.htm?ItemNumber=74009171&CustomerNumber=IA242E&StoreAccount=" rel="nofollow - http://www.allpartsstore.com/ItemDetl.htm?ItemNumber=74009171&CustomerNumber=IA242E&StoreAccount= 
  
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