Sky;
This depends on how you intend to drive the car. When doing these modifications, you need to take a holistic view of everything that effects breathing and flow. While the porting/polishing will improve flow, if you are keeping the same exhaust system, carbs, cam etc I don't know that a header in itself will add much to the performance. It may be more noisy, and being bolted with the intake, header flanges must be the same thickness, or you'll have to be shimming to get the mounting bolts to pull up correctly. For street driving, IMHO, the stock cast exhaust manifolds are not much of a hindrence to flow.These are torquey motors in stock trim. For track,a header with an entirely different free flow exhaust system would be better, as well as weber carbs.
Even with everything else left stock, you may find you'll need to enrichen the mixture a little due to the improved flow from the porting. If an uprated cam has been installed, you may have to replace the carb needles one richer.
Good Luck!
Pete Haburt
61 3000Mk1