My 74 boler
appears to have the same style frame.
I havent experienced any cracking yet but if I did I would do a couple things before even considering replacing the whole frame.
1) I would take a ball peen hammer and hit suspected areas with it to see if there is still strength in it.
2)drill a couple small holes around the cracked area to see if it still has some wall thickness
3) drill a hole at the end of the crack, grind out and weld it up.
4) make 2 gussets, one that goes up close to the floor and picks up the original gusset on the inside of the frame rail and one that is shaped to the bottom of the frame rail, welded to the gusset and out side edge of the rail.
Use it and check repairs.
I bought an axle
from Paul on this site, was a bolt in and works great.
Good luck with your boler!