what truck are you working on and what tranny are you trying to replace? I replaced the SM420 in my 59 Suburban with an SM465. I had to shorten the driveshaft since the 465 is longer but there were no issues with the cross member.
With the SM420 they made a few small changes to shift forks and if you change top covers, you can run into issues unless you stay within the same year group. Not sure on the 465 though.
