I just got a FBB-38 a few days ago, and with my batches of Sickles:
1/0 will barely fit with the point tight on the indent wall, 2/0 needs a slight mold trim where the hook point contacts over the outer tip of the indent, and 3/0 will kinda fit. By "kinda" I mean 2/0 & 3/0 shanks aren't dead center straight thru the collar, but will probably pour OK.
Single strand weedguard mod looks possible, but the guard will have to sit on or very close to the hook point. With guards as long as in the pic jig, could only pour every other cavity at a time.