Sure, stun will work fine in this case. There are number of different options
1: stun basic - encaps everything from the sdlc into tcp, tunnel from router A to router B, decaps back into sdlc on the remote
2: stun sdlc - match on the configured sdlc address and send to remote router, this allows for more complex scenarios (send sdlc addr c1 to remote B and sdlc address c2 to remote C etc).
3: stun sdlc local-ack - as per #2 but also locally-acknowledge control frames and only send data over the wan
You only have one tcp connection between a pair of stun rouiters, but you can tunnle multiple sdlc connections. These are identified by sdlc address and stun group. eg: config for one peer
stun peer-name 10.48.64.26
stun protocol-group 2 sdlc
!
interface Ethernet1/0
ip address 10.48.64.26 255.255.255.0
!
interface Serial3/0
mtu 2104
no ip address
encapsulation stun
serial restart-delay 0
clockrate 64000
stun group 2
stun route address 11 tcp 10.48.64.18
Cisco have DCE (clock) & DTE (clocked) cables for x21, v35 and v24 (I think this is db25). If you want the router to clock the connection then take a dce cable and configure clockrate on the interface, otherwise use dte.
Stun is well established but I suspect is not widely deployed on newer platforms (29/3900s). There might be issues with some of the newer WIC card simply due to lack of exposure.
Matthew