Well ;
Don't you have another old SM420 to yank the rear housing off of ? . that crack is a serious thing , you cannot have it brazed , I'd not even want it properly welded .
These are REALLY sturdy trannies , I say fix the back end and slap it in but then , I'm not the one humping the 350 # of cast iron....
Don't forget the guide pins ! .
The input shaft wiggling is normal , this is why it's so important to ALWAYS remove the clutch and replace the pilot bearing in the end of the crank shaft . easy to do and cheap too , it'll improve the shifting greatly by holding the input shaft in proper alignment .
Plus , you'll be able to check the clutch disc , clean it of oil or replace as necessary , clean up the flywheel , I know it's greasy and dusty .
The U-Joint can be swapped ~ there are two types , light duty and heavy duty , use what you have if the splines match .
It's been a while since I had one apart but , IIRC , the speedo drive gear slips over the splines & you re use it , be sure that front yoke bolt is tight ! .
As you're there alone , you have to make the call but , looking for properly fitting parts ,matching hardware and all gaskets etc. as you re assemble it and you should be fine ~ I learned this by doing old farm trucks in the mud , using parts culled from junkers sitting way out in the lower 40 fields... I'm no genius so I believe anyone can do it if they but pay attention & work carefully and slowly , if you see a boo-boo , stop and go back , take it apart as necessary to correct any little thing like leaving the ball housing retainer off (oops) .
Remember : if you have a seriously dead part , like the cracked rear housing , Joe has plenty as this is a non wear part and there's no shortage of SM420 Parts lying around ~ ALL old junkyards have several that were taken apart , the Hot Rodders also have them cluttering up their garages , don't be shy to offer $35 for the perfectly good takeout tranny , NO ONE else will ever come calling for it .
You can do this ! .
