Usually when these engines come apart the bearings stay on the shaft.
Hopefully, and they haven't been spinning on the shaft....
If your new bearings are a tight fit on the shaft, I think I'd prefer to assemble them that way too
- that way you can be sure the bearings are the correct spacing from each other, and if they are tough
to get on then you can work on them - you don't want that while trying to poke the shaft through
them if the bearings are already in the case.
With the bearings on the shaft, you can heat the cases and slide the whole lot home.
Isn't the saying if you spit on the cases and they spit back then they are hot enough.
This should make them a slip-on fit.