I put headers on my '73 Blazer which of'course has no computer. I used the exhaust system from JC Whitney. It didn't bolt-in as advertised. It required several extra S-shaped pipes from AutoZone to even fit. And it is still going to need to be welded up to stop the exhaust leaks.
On the emissions stuff I have a '93 Mustang with a computer, aftermarket (long tube) headers, an air pump, and no cats. Everything I say is from my Mustang experience but it might relate your Blazer. There is no problem with the short style headers, many are even emissions legal. With the long headers there is a concern that the O2 sensor has been moved further down the exhaust stream and that could cause it to be cooler than normal and cause your car(truck) to run lean. My personal experience my car runs great on the highway (24-25mph with a 340hp V8) but smells like gas around town (but I do have a loppy cam that could be the cause of that). I have never seen any evidence that the O2 sensor relocation does or doesn't cause a problem.
'73 Blazer, 4" lift, 33" BFG Muds
