Can't upgrade 3GS 4.2.1 to 5.01!

[themagician8]

Want to upgrade my jailbroken 3GS 4.2.1 to newer 5.0.1 but iTunes gives me problems.
I was getting 1600 and 1604 errors. So I turned off my internet security.
But It still won't let me upgrade. It starts but then hangs after awhile, then it puts phone in recovery mode, and I have problems restoring as well! Thank God Redsnow gets it back to normal.
So I tried to restore to 5.0.1 with a custom ISPW made with Redsnow. But that doesn't work either.
It just puts it into revovery mode with a 1604 error msg.
I was reading things about SSH blobs and bootroms etc.
Apparently mine are to new or old or something.
(I don't really understand this stuff)
My IOS is4.2.1
My baseband is 05.15.04-6.4_M3S2
My 4th & 5th serial digits are 51 (forgot what that means...if that showed bootrom or ssh blobs but apparently that to new...? I dunno.)
The things I read said something about using tiny umbrella to fix the ssh blobs but I downloaded it and have no clue how to do this.
Also after trying to sync my my phone proir to upgrading, It seems to have filed up my phones HD and there was no space left. Don't know why! I had 1.5 gigs of space left before sync, nothing left after! I deleted a bunch of old apps in iTunes no longer on my phone and resynced, but stil only have 254 megabytes left!

What am I doing wrong, why won't iTunes let me do a legitimate upgrade?
Why can't I do it with the custom redsnow ISPW?
All I want to do is upgrade from 4.2.1 to 5.0.1 ... then jailbreak it.
How can I accomplish tis?

Thanks!

[iYeow]

Don't need to use custom fw as your bb is already not unlockable. Trigger iphone into recovery mode and restore to stock 5.0.1.