You'll have to experiment. Every softbox is different (70cm Rotalux?) and there's not much room inside. On the quad-bracket, the guns sit a little forward - the bracket is adjustable for this, but when pushed back they'll touch the sides of the softbox. You may have to dispense with the inner diffuser - experiment. With the guns pointing back, coverage is more even but you'll lose some brightness, pointed forward, hot-spots. Using the wide-panel evens things out, Stofens too, but both lose light. TBH I don't worry
too much about those, as I need brightness. Minimise hot-spots for sure but they don't actually make a huge difference to the quality of the light - you'll notice them in catchlights, but probably not otherwise unless working close. I position the guns so they bounce off the sides at a forward angle, flash head on widest zoom setting (without wide-panel) for the best evenness/brightness compromise. The smaller the softbox, the less options you have.
It's not an ideal solution. Speedlites are brilliantly versatile and can be used for pretty much anything - and that suits my varying needs well - but faffing about with adapters and brackets etc with multiple guns in a softbox is a PITA (pretty much impractical without radio remote control) and results not always optimum. If I was starting again, my flash outfit would be very different given the new options now available (Godox and others) but for
occasional location work when you need to raise the power with speedlites, or need HSS or auto-TTL etc, it's a doable workaround.
I'll also add, studio stuff is one thing, but location work quite another when distances go up and ambient light is bright. I never have enough power with speedlites and recycle times can be too slow at max output. I did a location job the other day with four YN600-EX-RT guns - two in one softbox as keylight, one in another, and the fourth lighting a more distant background. I got f/8 at ISO400, but recycle times were slow and I really wanted f/11 at ISO100. If the ambient was brighter, I wouldn't have been able to raise ISO and get away with it. As it happens, I could have had mains power on an extension lead and used studio heads, but didn't know that beforehand. It was one of those suck it and see jobs so I took speedlites that I knew could be pressed into just about anything. The client was delighted and I was pleased too, but I know I could have done better and certainly a lot easier with the right gear - it's less stressful too, helps the session run more professionally, and allows you to concentrate on the subject better. I was lucky to capture an impromptu moment in one shot, as it was gone before the guns recycled and of course that's the one that's going on the wall! In fact, just writing this has decided me to change a few things. Not enough time or money before the next speedlites day out next week haha. Sorry to waffle