The adapter / coupler that attaches the Acme rod to the stepper ( = collar on the bottom?) is the main problem with this approach. If it does not line up the center of the motor shaft with the center of the threaded rod, you are right back to troubles. That is where getting an integrated stepper / rod really shines (there is no possibility of offset). Depending on where you buy the parts, it may actually be just about as cheap. It gets the couplers out of a very crowded part of the printer andmakes the conversion easier.