Well... I have taken a few of these apart. It will be a dovetail joint and it will be glued together with an 'epoxy like substance'. The glue is the killer. I've personally destroyed four or five FG-75's from e-Bay trying to get the joint to come undone. The glue will release with heat, but using steam turned out to be not such a 'hot' solution. The wood soaks up way too much water before anything is warm enough.
I've made heat shields from cardboard and aluminum foil and you c an use several different shapes of these around the joint to protect the neck and the sides at the neck block. The heat source can be a small blanket, a hair dryer, etc. Now, the thin line to walk is loosening the glue before the finish bubbles! 
Is there a better way? I think so. I love the old Yammies. I particularly like the tone that comes from the FG-140 which was Yamaha's version of the D-18. I've come to the conclusion that the only sane neck reset involves using a Vet's bone saw to saw (a thin kerf Japanese saw would work as well) through the joint. The neck can then be easily converted to a bolt-on. No harm-no foul. If done carefully, the finish isn't too bad to patch.
The thing to remember is that these guitars are 'players' -- they make great boat/beach guitars. Snag yourself an FG-75 from the Bay for $50 and practice the cut. Have fun -- it ain't a pre-war Martin that you are operating on! 