In lines 83-93, what evidence is given to make this proposal sound practical? A modest proposal

Please quote these lines so I know exactly the ones you mean. Counting lines can produce a different number depending on where you are reading it.

Infant's flesh will be in season throughout the year, but more plentiful in March, and a little before and after; for we are told by a grave author, an eminent French physician, that fish being a prolific diet, there are more children born in Roman
Catholic countries about nine months after Lent, the markets will be more glutted than usual, because the number of Popish infants, is at least three to one in this kingdom, and therefore it will have one other collateral advantage, by lessening the number of Papists among us.
I have already computed the charge of nursing a beggar's child (in which list I reckon all cottagers, laborers, and four-fifths of the farmers) to be about two shillings per annum, rags included; and I believe no gentleman would repine to give ten shillings for
the carcass of a good fat child, which, as I have said, will make four dishes of excellent nutritive meat, when he hath only some particular friend, or his own family to dine with
him. Thus the squire will learn to be a good landlord, and grow popular among his tenants, the mother will have eight shillings neat profit, and be fit for work till she produces another child. These are the lines