A conversion neck is one that can convert an instrument from one scale length to another with no other modification. A common conversion is 34" to 32". By overhanging the 20th fret, or leaving it off, you can use the stock bridge location on the body, just adjust for intonation. Such a neck will work on any standard bass body. A conversion from 34" to 30" requires overhanging two frets, or losing one or both.
Although sold as a hardwood, poplar is actually softer than some pines. It often has an unattractive green color, although the grain patterning is usually nice. The grain itself is tight, requiring no filling. Poplar is suitable for stained and solid-color finishes that hide the natural color. Poplar requires a hard protective finish like polyurethane.