One thing you have to learn as a mom- and it doesn't matter whether you breastfeed or formula-feed!- is to not judge your success as the mom of a new baby solely by how "content" that baby is. New babies are fussy, fussy, FUSSY creatures! Fussiness is just normal for them. It's not an indication of you doing something wrong, or your milk being somehow "bad" for the baby.
If your baby is gaining weight well from nursing alone, then you are doing everything right, no matter if that baby is a 24/7 fusspot.
Fast letdowns can increase a baby's fussiness, because when a fast and heavy letdown begins, it's like the baby is drinking from a hose that has suddenly turned on full blast. Often fussiness comes just a minute or two into the feeding, around the time baby gets the first letdown or gets so full that he's not interested in continuing to eat. The first thing to try is to adopt reclined feeding positions, since reclining enlists gravity to slow the milk flow to the baby and make feeding more comfortable.
Block feeding is something you want to do only if you are sure that you have oversupply. If you can share some of what you're experiencing- in terms of engorgement, feelings of fullness, how strongly you feel/see letdowns (you might see milk squirt/stream from the breast when baby pulls off), whether or not you're pumping and how much milk you get if you do- we can help you figure out whether or not oversupply is part of the issue for you, and whether or not block feeding is a good idea.
When it comes to weight gain, there's no such thing as "too big" when it comes to a breastfed baby. Breastfed babies can't be overfed because they learn to read their own satiation cues and stop the feeding whenever they feel full. But they do demonstrate a very different pattern of weight gain from formula-fed babies. Formula-fed babies tend to gain weight rather slowly at first, and then increase their rate of weight gain as time goes on. Breastfed babies tend to gain weight very quickly at first- as much as a pound a week is not uncommon!- and then slow down as they get older. Both formula-fed and breastfed babies tend to reach their first birthdays at around the same weight (statistically speaking), but the formula-fed babies are the ones who have a lifelong increased risk of becoming obese.