I tend to agree with the other posters, although, I haven't worked out the exact figures, as, ceeaton has, also, I tend to agree with norcal, in that you will end up with a very strong alcohol content, which is basically very sweet, this used to be fairly common practice, I have some vintage wine making books, which, tend to stipulate excessive amounts of sugar, as norcal suggests, this was in order to retain some sweetness, after final ferment has finished and after the yeast has exhausted its ability to convert sugar into alcohol, there are inherent dangers in this practice, such as fermentation re starting after bottling, also, it can stiffle the yeast, into inactivity, so, adding the sugar in batches is very likely a good idea, making sure you keep a record of how much you have added, for your own records.
You can, also, divide the amounts down , if you want to make a test batch, so, for example 5 gallons, would need 33 1/3 pounds of sugar, using your grand fathers recipie, as an example, which, thinking about it, is actually a ridiculously high amount of sugar, as, a general rule I personally use around one kilo gram or about 2.2 pounds of sugar per gallon meaning, around 11 pounds in 5 gallons, which is probably a good ball park figure, for a dry wine, the recipie your grand father was using, will definitely make a very sweet drink, which may have been his preferred type and was fairly normal one time for country style wines.
The family friend who gave you the scrap of paper, might be an interesting person to chat with, for example, red or white grapes ? how your grand father sourced them (eg) shop bought them, or, obtained purely from their own land, friends etc, once you have that knowledge it should make it easier to retrace his wine making steps, then, when your wine is finally made, maybe have a glass or two with them, it might also, transpire he used differing sources, depending, on the availability of stuff.