The tests above were done with the manifolds having 2" pipes but the headers were run through the collectors with no mention of any pipes after the collectors. Bigger down pipes can change the manifold numbers, and restricting the headers after the collectors could change the header numbers as well.
The OP has larger exit manifolds, larger pipes, and is adding a crossover pipe. I don't see an issue with the exhaust causing a problem with a mild stroker. Sure, more power can be had with the headers, but all the header bs comes with them. You should be able to put together a pretty stout street small block with the Magnum heads and the exhaust to are planning. Have fun!

For years we ran LA heads with LA exhaust manifolds and big down pipes in our cast iron exhaust restricted class on the dirt track and we did pretty well in the class. We ran a pretty big cam with those LA manifolds. When we were finally able to use headers, that little LA really ran, but so did everyone else. Gene