vim: how to gq a visual block?

as in subject, suppose we have a gq'ed block already:

0 1 2
12345678901234567890123456789
lsdjflsdlksjfsdlfjsld 123
lsjfsdfksdjfldsfsdlsl 456
lsjdlfsdlfjsalfslfsjs 789


now I want to re-format the character blocks , for example, extend them
from current length 21 to some other length, say, 25, or squeeze to 18,
without affacting the "digit blocks".

is it possible at all with any command or plugins?

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