Can anyone explain the legality of building and selling right side plates for the 58?
Depends. "80% blanks" can be made and sold without a license, but you have to submit a sample to ATF for determination they they aren't far enough along to be a receiver. The 80% thing is just marketing hype, ATF decides on each one where the "not a receiver" versus "is a receiver" cutoff point is. Basically you make one and send it in, then based on that determination go up or down in the machining steps and resubmit until you get just below the "is a receiver" point. Be aware ATF has been known to change their minds on these and declare them receivers at a later date.
The alternative is get a manufacturer FFL (type 07) and go through the expense and paperwork of setting up shop as a licensed manufacturer. IIRC, you also have to pay big bucks to the state department to register as an arms exporter even if you only sell domestically. Liability insurance premiums as well.
You would also have to come up with or use an ATF approved semi auto conversion, these usually entail raised sections on the inside of the sideplate that would exclude unmodified FA parts from fitting inside. The semi parts would be slotted to clear of course.