I actually prefer the Stainless T-Nuts like these ones . I have cracked a couple of brass ones. Plus they are easier to get in and out. The big brass ones are good but like you found they are tight in some boxes but don't be afraid to grind them a little to fit (the nuts not the box)

The S/S nuts in the above pic have no flanged sides on them , sure they will work ??????

Piros: Always on the money. Thanks. I managed to "wedge" 2 of those chunky copper t-nuts in the rear of the stand up tracks but used slimmer ones for the front. Couldn't get any of the chunky ones into the prone board tracks so used slim (standard?) copper ones there. But now you have guided that stainless are OK, I have a bunch of them...so will maybe switch now.

foilSURFmachines: I sourced my "correct" brass t-nuts from a windsuring store....so yes, completely agree.