The Baker six speed wont drop into a XB engine. When I get back from Iraq I'm going to work on it.
The XL and Tubers use a trap door transmission. What that means, is it isn't necessary to split the cases to get to the transmission. Once you remove the primary case, there is a "trap door" on the case that supports the input bearing and transmission. This door unbolts and allows removal of the transmission.
In the Baker six speed, this door is replaced with a modified Baker door that allows room for the Baker sixth gear.
Keep in mind that in XL transmissions the shift forks ride on the shift shaft and not on fork shafts. I believe if I use the forks out of a blast transmission and remove the left bearing area of the XB transmission machining a flange for the trap door I can make it work.
Edit: 1125 transmission, not a chance.