That is one lotta stems I see in there. Well, maybe not for some but for me -- I don't allow any, they'll make things pretty puckery down the road. That may just be my experience, though.
I think you'll have issues. Is there any way to get another one and split the must between the two? Or, a Rubbermaid BRUTE* grey garbage can, 32-gallon with cover, was only $26 at my local Lowe's just about 5 or 6 weeks ago. It's a perfectly acceptable fermenter and will give you a LOT more cushion than you've got there. Also if you have any food processors or import/export businesses around you, they may be willing to give or sell you containers that held olives or olive oil or whatever (non-toxic) other ingredients; I've got a 50-gallon one with large screw-on 2-piece lid that's terrific (got it from a guy that got it from a local importer, it held olives from Italy!). No odors, and it's perfect as a wine primary fermentor.
The cap will definitely rise, especially once fermentation is underway. Any cloth you have covering the bucket will almost certainly come in contact with the cap and will become "juiced" enough that any fruit flies landing on the towel or whatever will only have to lay their eggs on the cloth, without needing direct contact with the must, and the maggots will hatch and migrate through the cloth into your fermenting wine.
The good news is that you only need the primary for a week or less and then you'll be pressing the pomace. The bad news is that you'll need the primary for about a week or less and you have to punch the cap down at least twice a day until you press the pomace.
Even if you cover with the lid, I THINK you'll still have a lot of pressure pushing against the lid from the inside. 6-8" seems like a lot of room now, but when things get going with the ferment, it MAY not be enough. I know that it isn't with me and my grapes, but what I do isn't necessarily going to be your experience; I put all my grapes into nylon pressing bags, as many as I need to hold the grapes (sometimes before crush, and crush them in the bags, sometimes crush and then pour into the bags that are suspended over the fermentor -- either way, the pulp is in bags and the bags are tied closed and placed in the fermentor), and that may account for why I need a LOT more headspace than that (the 6-8" you've got) since the bags may force the pulp to have a lot more bouyancy than yours will have. I confess, I have no idea on that one since I've only ever, always, used bags.
On the other hand, others may join in here and tell you (and me) that I'm more full of crap than your fermentor is full of grapes.
I do have to say, that's a really nice picture, isn't it? Congrats on your bounty of grapes! You've gotta be stoked... especially being your first ferment! 200 freakin' POUNDS of grapes!!! Start asking around for empty bottles NOW, lol!!! You're gonna love it. BTW, what kind of grapes are they? And do you have a recipe that you're following?
Hopefully others will chime in. I wish you great success!
- Tim