Unless you are planning Dukes of Hazzard jumps, the small block will survive just fine without the extra support brackets. I ran a lot of SB Mopars with an auto on a dirt track, all without the extra brackets, and never had a single issue. I can almost promise you are not going to put your Dart through what those old dirt track cars went through.

As to the 904/727 deal. If both are box stock transmissions, the 727 will hold up much better under abuse then a 904 will. If the trans is being built, then the 904 can probably hang with a box stock 727, but if both are being built, the 727 is a very tough trans and can take a lot of abuse.
I tend to kill auto transmissions. Its 727s for me. I've killed way more 904 then I ever killed 727s, and I've had more 727s.

Of course, your mileage may differ.