I installed ATR headers on my 87 GN and based on my mechanics advice (he is a Buick guru here in OK) he advised that in order to make a 3" down pipe work with the headers I should buy a THDP made specifically for ATR headers. I did and do not have any fittment problems, rattles, or contact with control arm, heater box, etc.........By the way, the powder coating will burn right off a down pipe....even if it's high temp powder..the best thing to coat headers and a down pipe with is a ceramic coating as it will withstand temperatures as hgh as 2000 f.