Personally I'm not sure why there is a desire to install a performance header but retain the stock restrictive exhaust system. But I'm not questioning anyones preferences, just making a general comment.
If modifications are required to install the stock system onto the header then perhaps look at the difference in cost between having that done and having a new system built specifically for it.
It's OK to question my preferences, as long as you realize it's OK for them to be different from yours. It's a legitimate question. The answer lies in understanding the starting point and the goals.
Starting point:
After 40 years, my muffler has holes corroded in it - it was time to be replace it.
The catalytic converter flange with the flex-joint broke off. Looks like the flex joint was bolted up too tight to move and corroded in place. I welded it as a temporary fix, but recognized it was time for that to be replaced, too.
The condition of the downpipe, prior to the cat? Who knows - but it's 40 years old.
The manifold? That chunk of cast iron isn't worn out, but internet wisdom tells me it's a significant restriction.
Another aspect of the starting point is that I don't have lots of time to fool with this, so was looking for a bolt-in solution. (That's not a complaint. The other things I'm giving higher priority to are good things which I am thankful for. But it's a factor I had to consider.)
My decision process:
The available long-tube headers take us all the way from the block to the muffler. They eliminate the restriction in the exhaust manifold, eliminate the downpipe (of questionable integrity), and eliminate the catalytic converter with the broken flange (I wrestled with that decision). Of the two versions readily available, one was Stainless Steel, and claimed to work with either of the two stock mufflers or a 'performance' muffler. Most 'performance' mufflers are louder than stock. I put a higher priority on the muffler actually muffling than folks who would choose something else. Part of the reason for that is to have a higher likelihood of someone occupying the passenger seat. That makes more difference to me than extracting an extra horsepower or two. Of the stock mufflers, the FI Muffler ended in the right plane to make the brace to the transmission line up with the joint, which seemed desirable. Fortunately for me, forum-member Budzinsk had a NOS FI Muffler for sale at a good price.
That's how I got where I am. I recognize others would make different choices, and I'm OK with that. How I get from here to an installed system will bring more choices, and I'll start a workshop thread for that.
Brian