I'll try and clear this up, I sense some frustration, and I apologize for that. When this all started (my desire to swap trannies) I also read all the info available and had to sort out what I needed. Since that time some of the sources have dried up, and new ones have appeared. Please be patient, this isn't your average swap.
Here are some additional resources for Info and parts:
Great pics and procedure stuff from PinHead
http://members.cox.net/cglabe1/SM420/sm420.html
Classic Cruisers, parts can be bought from them
www.classiccruisers.com
The gm pilot bearing part # is 14061685, this was posted on my web page for this conversion, along with source info for the bearing and other items.
The bushing is needed to support the shorter input shaft of the sm420. It is not weaker, nor will it fail if made from mild steel. The bearing is a roller type, better than brass bushings.
If you have access to machine shop equipment, you can make a bushing from oilite and skip the bearing. Just measure the input bearing surface of the tranny and crank hole for the pilot. I suspect that is the dimensions posted above, but since mine is installed I have no way of confirming this.
If Pinhead see's this he did that exact thing, and may be able to offer specifics. If Pablo works it out, you should make a more than a few and sell em. Same with the sleeve for the throwout bearing. I bought both of these items from Wardens while they were in business. Not worth the hassle to make.
I'll continue to watch this thread for more questions.