You can make a bow of that lenght and weight with rawhide fine, pending knots, tiller, etc. The wood will do it with a rawhide backing. Get the back as smooth as you can, but like has been mentioned here lately, it's tough. The rings are very thin, but there is a bit of coloration difference that will help you out. You'll start to notice it on the first three or four rings that you accidently gouge through.
The pure heartwood idea sounds really neat, but it will be tougher still to get a solid ring. If you do get a bow out of that, be SURE to let us know how it goes.
Incidently, my 66" ERC bendy handle is in the shop getting a touchup application of TB3. After shooting last night I noticed that the rawhide was starting to seperate a bit along the edges. Probably from my keeping it indoors with wood heat all winter and not keeping the linseed oil on it often enough. I'm hoping it comes through alright. If not, it shoot thousands of arrows and took two deer this past season, so it doesn't owe me anything.
Good luck!! ERC is pretty wood!