Confusion.. Please help

[aaddaam]

Guys i have an Iphone 3GS on 4.0.1 with 5.13.04 bootroom.. its an MC model with new bootroom and i have SHSH blobs saved for 3.1.3, 4.0, 4.0.1 and 4.1
now i also have an AT&T sim card which i got from a friend.
my question is that can i update to 4.1 using tiny umbrella while preserving my baseband? and if i upgrade using tinyumbrella than would i be able to activate my iphone using that At&T sim card? i rely on ultrasn0w carrier unlock..
is it all ok for me to upgrade to 4.1?

[zoolman88]

No that only works on iPhone 4s 4.0.2 is the highest u can go and still have a unlock but I would stay on 4.0.1

[aaddaam]

so the only option left is to ask a friend having mac to cook custom firmware for me using pwnage tools?

[Simon]

Yes a custom firmware is your only option