wow Zeus thats an original headstock shape, wouldn't want to go any narrower in the centre area as there will be quite a bit of string tension on D G B E strings. Should look cool once it's cleaned up.
Walnut timbermate will work well.
To prepare it I use a flat surface to prepare it, old icecream lids work well. Add a bit of water and mix it with the TM till you get a fairly thick slurry. Can be applied with a plastic paint scraper or a sponge/squeegie. Wipe it over the entire body and let it dry for a few hours before sanding back. THe only timbermate left after sanding will be sitting in all the small pits in the grain surface. A bit of practice you will work it out pretty quickly.
Worth pre-sanding to at least 240 grit before filling