I don't know if this is any help to you but I happened to do something similar a few weeks ago for a UK customer, albeit with 2 x E1's. FYI the Telco provided me with a sample config so I recommend that you speak to yours to clarify the position.
1) I used a 2801 with the VWIC 2MFT-E1 - you shouldn't have a problem with a T1. I used 1 x existing 3640 for my config.
2) The Telco configured their end at the same time I did mine. They stated that the Multilink had to be done this way as you can hit problems if you do them out of sync.
3) My config was as follows:
Note that the Serial interfaces are assigned to a Multilink group. The Serial Interfaces have no ip address. The Multilink interface has 1 x IP address that is for the Multilink group.
! controller E1 3/0 framing NO-CRC4 channel-group 0 timeslots 1-31 ! controller E1 3/1 framing NO-CRC4 channel-group 0 timeslots 1-31 ! interface Multilink1 ip address XXX.XXX.XXX.XXX 255.255.255.252 no cdp enable ppp multilink ppp multilink fragment disable ppp multilink group 1 ! interface Serial3/0:0 bandwidth 2048 no ip address encapsulation ppp no cdp enable ppp multilink ppp multilink group 1 ! interface Serial3/1:0 no ip address encapsulation ppp no cdp enable ppp multilink ppp multilink group 1 !