I did the two piece split thing on my 261 and I have been running it for about twelve years with no problem. The disadvantage is they are hard to install when you have an intake manifold and two exhaust manifolds to install at the same time. Not enough hands. I cut the heat box out completely and moved the cast iron exhaust flange to the bigger end of the runner between the ports. Then cut a cast flange off another manifold and put it on the other runner. I installed dowel pins into the inside edge of the flange where the manifold meets the head. And drilled short holes in the head for the dowels. This helps hold the inside ends in place.
If you do your own, Good Luck.
