They'll fit without any problem, unless the floorpan in one or the other cars is rusting. If the donor 300D is rusting, make sure you clean everything well so as to not contaminate the 230E with oxidation.
My dad swapped a front passenger-side seat from a junker into the driver's side of his '77 240D. I hate driving it now because the replacement sseat doesn't have the armrest!
'83 240D "Steiner"
'00 New Beetle TDI