5lbs in wood is fine for a 64" wingspan warbird, you might want to sand it down to the glass and lighten some of that filler, and repaint it.
looks as though its ready for retracts, if you want to install the flaps use 1/16 ply to make the split flaps,if you can't find the sterling instructs , Top Flight has down loadable sets, so you will have a idea of how to do them.
there not the exact same but close enough to give you a good idea of the method to use for split flaps on the spit.
another thing is engine size your going to need at least a 61-75 2stroke or a 91-120 4st engine to fly that plane with retractable gear ,engine,radio,and all the misc stuff to go along with it your looking at a 8-10lb plane,its going to be fast.
here's what I am talking about my 11lb Top flight Spitfire powered by a OS 91 4st,retracts,and all the goodies,its glassed with water based poly and 1/2 oz cloth.
and my #1 assistant for scale