Ok ive asked this before i didnt really get anything back so lets try it again since i dont want to brick my phone leading to somebody telling me i shouldnt have done it this way etc...

What I have:
3GS
- Old Bootrom (***21***...)
- 3.1.3
- 5.12.01
- Jailbroken by Spirit. Sadface
- AT&T Provider (not worried about unlocking)
- iTunes, 9.2, 9.1, 9.0 and probably 8.2 on my old PB.
- A downloaded fresh install of 3.1.3, or any version i may need.
- Cydia seems to only have SHSH on file for OS 3.1.3 (thought i saved 3.1.2 as well but not seeing it)

So, given the situation i can only go as far back as 3.1.3 due to shsh's and modem firmware. Should I use itunes 9.1 (or does this not matter?).
I will need to use sn0wbreeze since it will JB my device in 3.1.3 and (not sure on this, because the main site has instructions to download spirit and not sn0wbreeze) its not Spirit.
So hopefully i should be able to restore (shift clicking) and select the 3.1.3.ispw to install back onto my 3GS, then JB with sn0wbreeze, and then follow the steps by devteam for the pwnagetool4.
Am i missing anything or is this just not possible?