I said a long time ago that I was going to stay out of header discussions, but here I am....
I've owned street cars with headers. The last one was an A12 Roadrunner and I ended up switching to exhaust manifolds.
IMHO...
First, they can be a PITA... They are more prone to leaks and getting caught on things. I once caught a rock while backing up a car I used to drag race and bent the crap out of one... Those $$ Stahl adjustable headers weren't cheap to replace. A friend caught one backing it off his trailer (another $$ Stahl header too!). I saw one street car get a header caught on a railroad track. That took all the exhaust on that side out.
I've never installed headers in a C body, so I can't address that, but they can be challenging to get them in right. Then everything you do has to be worked around. I had a small block powered B-body that needed the right angle oil filter adaptor to get them to fit.... Not sure if you need that with the Poly motor or not. Changing the starter can be a PITA with some headers too.
Second, and this always twists everyone in a knot... Performance wise, you may not really notice a big (if any) improvement. Let's face it, you are looking for low RPM torque in a quick street car... Probably under 4500 RPM. You aren't going to want to twist that Poly that much.... It is a 56 year old engine that I'm assuming has never been apart. Think valve springs. The headers aren't going to make that much more power until you start hitting ~4500 RPM. The rest of the engine is going to start laying down about then.
Most of the gains I hear about are "seat of the pants" and that usually means "it sounds like it's making more power".. You don't usually see what the ET drop is or dyno pulls "before and after".
If you were building this motor/car with the right combination to use the headers, I'd say "go for it", but reality is you have a stock cam, a stock torque convertor and a high rear end gear (low ratio). To take advantage of the headers, you really need to address a lot of stuff here before the $$ adds up to HP and torque.
It is your car... Do what you want... Just think about it and don't believe all the hype made to sell you headers.
If it were me, I'd do the TTI exhaust (I have that on my '70 and they are really nice), swap to a 4bbl and look at having the distributor recurved. Maybe a swap to 3.23 gears.. A cam change might be in order too. That will give you a nice running car that you can enjoy driving.
