"if I just had a good way of checking the xth bit of a number without building a bitmask (unless someone can clue me in on how to do that quickly)"

Don't know if this is what you're after, but it's a quick way to generate bitmasks on the fly.

Two to the power of the number of bits to include in the mask minus one, left-shifted by the LSB

perl -E '$bits=40; $lsb=3; printf("%b", ((2**$bits)-1) << $lsb)'

1111111111111111111111111111111111111111000
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Don't left-shift at all if you simply want the mask for the number of bits you're masking exactly.