Splitting work to pass through the integer overflow error

[penguinkeeper]

But why does the keyspace have the size of the mask? Shouldn't it have the size of the -l option?

No because Hashcat still has to calculate with it. It's just a safety feature. The easiest way is to just have the final 2 characters be static, so you do:
?a?a...00
?a?a...01
?a?a...02
etc

but my advice that this will still take probably tens of years to exhaust still exists. Just because you *can* run it, doesn't mean you *should*