I am sure you can use a belt. Just make sure it is no more than 4 inches thick (top to bottom) because most feds dont allow any bigger. It is good to see you dont use a belt yet your core will thank you. I didnt use a belt until a few years ago. I started with a crappy nike belt and now I use a lever belt which is great because it is easy to get on. A belt will help you lift a few more lbs but I use it mostly for spine protection. I strained by low back 3x doing deadlifts. That is why I started using a belt.
On the diet yeah mine as well up the calories so you can be a real 198lber. It should help your squat and bench. Not neccessarily the deadlift. From my experience my deadlift suffered from weight gain but my bench and squat got better. I Deadlifted 545 at 181 and now I am 190 down from 196 and I haven't gotten any better at deads.
Discipline makes you mentally and physically tough!
Currently making everything stronger!!
Once I stick to the program I am going to get really strong.