Lets not get to caught up in the moment.
My vote is safety.
If the motor and trans are 'upgrades', then do the rails.
If the motor and trans are 'required', then do the motor.
Unless someone more knowing can give you an option to stabilize the rails temporarily while the cash flow catches up to your needs...
There will always be another donor car....always. Someone grows tired of a project and doesn't want to put it back together, priorities change and have to sell a car for other bills, wants to clean out the barn to sell the place when his uncle died.... whatever reason ... there will always be another donor car.