Maybe I don't quite have the picture...
What I believe you started with is a slip joint in front of the bearing and a solid connection to the SM420.
If you use a slip yoke in your saginaw and keep the existing front shaft you will have two slip joints and could have problems as a result. But maybe not... It would be a problem if the shaft could get too short and fall out, or perhaps just wobble.
As long as the fully compressed length isn't too long, its never going to be too long. But I'd worry that it could be too short.
I'd look for a front shaft that does not have the slip joint from a truck with a 3-speed or an automatic.
