wow. I redid your math and their math because I didn't believe it.
on the alu you remove a bit threading it and you're done, and on the steel you weld a bunch of additional to it, so probably nearly same finished weights.
2.25*40 Al = 16.22#
2*40 Al = 12.87#
steel math:
40*(2^2 - 1.5^2)*.2904 = 26#
bah!