The 3speed transmission is the same as the 4 speed in the .1 series. The valve body is different. I can not remember the details but I remember that you just have to swap the valve body and then it is a 4 speed. I used to work with a guy that did this many times.
Also remember that the v8 swap in a 124 requires the crossmember to be repl and some changes in the firewall. You cannot just drop one in unless it is a original v8 car.