For the swap you will have to change frame mounts to work with the SBC clamshell mounts.
The battery and starter are on the driver's side for the 307 and both are on the passenger side for the SBC so you will need to buy or make new battery cables and possibly modify or extend the wiring for the starter.
No oil pan issues and you can run a 7 qt pan with kick outs like Moroso or Milidon with no frame or ground clearance issues (probably need a mini starter though)
Your 700R4 will require modification to your existing cross member or you can buy a new one.
You will probably want to upgrade the radiator also for the 400 and buy hoses for a gbody Monte.
You will need all of the accessories for a SBC nothing will transfer directly from the 307 to your 400
You can get a serpentine set up for your pulleys and it will put the AC compressor in a similar location as the 307 or you can run a stock v-belt set up but you will need the AC hoses and hardline for a gbody Monte since it is on the driver's side.
For my SBC for my Regal I already had aluminum v-belt pulleys from a donor car so I made brackets to mount the AC on the passenger side for a v-belt so I can use the factory 307 set up plus I don't like the compressor on the driver's side.
And lastly try to save anything you remove just in case.