I think, as Lee says, they would put them anywhere they would safely fit.
As a matter of fact, just today I primed my M9 trailer portion of this kit (it hit 60 degrees here in NJ today 
and as I’m nearing completion of mine, my references are handy. The main one being “M19-20 Tank Transporter - Detail in Action” by David Doyle. This book has numerous photos of Lee, Sherman, and M10 tank destroyers loaded, some of which don’t use the accessory chocks at all, some only just two. I haven’t yet seen a photo of an M4A3E8 loaded though.
A variety of “unloaded” photos show both 2 and 4 accessory chocks in various places on the trailer. There is even a photo of an M19 loaded with an LVT, clearly on the move, and no chocks in sight. They carried just about anything on these trailers, from LCVPs to stacks of boxed ammunition, Panther tanks, Churchills and so on. I’d use your imagination; regarding the ballast box, most photos I’ve seen show them full with just that, ballast. Anything heavy to improve the traction - large concrete blocks, 6 or 8 55-gallon drums (presumably full of something)……
Share some photos of your build!
Jimbo