A999/A500 Aluminum or Steel planetaries?

I intend on putting a 42RH behind a 5.7 hemi in my 66 Dart and I see listed on wittrans there are both 4 pinion steel and aluminum planetaries for both front and rear. The OD has the option for a 15* 6-pinion planetary, which the only one I know of from my research is aluminum. Is there any problem with using these aluminum planetaries in a street car? I read an article that said the later released A999 steel-style low gearset planets were actually weaker than the original aluminum ones. Thoughts?