I agree with your thoughts Bazyle.
Do hope he braces that hole with a centre strut, before he adds those heavy lumps of cast iron!
If it starts to sag then he will be in the doo-doo me thinks.
A smaller opening with roller shelves as per my lathe, however, would be ideal for storing all the cutters, vices, spacers and so on, but only as you point out, if space is at a premium.
Just a little re-organising/tidying up can release a surprising amount of useable space, as I have found out on numerous occasions, and are still doing so 
Geoff – Video Light Box finished today (hopefully)