When people say that 6 cylinder engines have "lots of torque", they are generally referring to the larger truck engines. The 200 cubic inch six cylinder has at least 100ftlbs LESS torque than a 289 V8.
Headers won't even get you close.
If you want to keep the car original, then I would just leave the 6 cylinder in it and put a header and a glasspack muffler and enjoy the car as it is. There is a lot more to swapping a V8 into a six cylinder car than meets the eye and if you take the time to do it correctly, you sure as hell aren't going to want to "undo it" when you sell it.
As far as a price comparison goes, you're looking at several hundred to do the exhaust system on your 6 cylinder and the cost of a V8 swap is going to be a few thousand dollars, at least.
Id look into a Doug Thorley header, if they're available, for your engine. Their designs are known for their low end torque.