Headers 1990 9C1

I am trying to find out which headers fit a 1990 Caprice 9C1.  But the issue I might have is I am looking for (1.75) 1-3/4 headers due to the fact I want to run a 400 plus inch small block engine.  I don't think that 1.625 (1-5/ or 1.5 (1-1/2) inch primary set will be able to flow enough to run the engine even to 5000 RPM.  Let alone 5800 or higher.  I don't care if it would be shortie, mid-length or long tube.  Then I'll worry about the rest of the exhaust later.  

Does any set from an early Chevelle fit or older Impala?  I have heard the 73 to 75 Chevelle Big Block headers would fit a Big Block conversion but would the same year Small Block ones work?

Most headers from second generation camaro fit, though I have found clearance to the passenger side frame rail can be really tight, and the frame to crossmember brace on the drivers side had to be removed on my car to clear the headers but I have the old style on my '79

Ok, so I decided to go with a big block.  Headers on order.  Will inform.