I had an original manifold that had been split probably years ago. It was cracking where the "new" elbow was added, so I took it to a good welding shop, and they pointed out to me that the spilt was never completed by putting a little divider into the manifold to separate the flow 3 and 3 (all the cylinders just dumped into one big space that had two outlets). They beefed up the welding around the add-on elbow, and put in a little piece of metal to split the flow evenly. Bottom line is: find a good welding shop with an old-timer (or younger guy, but usually it's an old-timer) who knows how to weld cast iron. It's an art, from what I understand. Can't say it added a lot of power (or any at all), but it sure sounds good.
