I've never - and I mean never - had any luck heating deflex out of a yew stave. It always comes back once you start bending it. A very small area is fine, as you can use steam or dry heat and pop the deflex out quite easily, but if the stave is deflexed all over, or it's a long gentle curve somewhere you need to think of a different approach to getting a good bow.
Not all yew is good for a classic English longbow. Wood is wood, and it's always different. You learn pretty quickly when you make self bows that you really can't force a bow design onto a piece of wood - you HAVE to let the wood dictate what happens. If your stave has lots of deflex, you'll need to accept that and work with it.
The risk is using steam or dry heat and cranking the deflex out, only to have it either break on you because it's been over-stressed the wrong way or the amount of heat required was too much (easy mistake when you're just starting out!) or to have all the deflex come straight back the minute it's on the tiller, only to find that you've roughed out the bow too close to final dimensions to get anything else out of it. The last thing you want is to have a deflexed yew stave with no extra thickness at the tips for initial recurving, and you'll just end up with a deflexed longbow. Not good.
Some deflex is fine, but it will get worse as you tiller (depending how experienced you are, it could get a lot worse!) and you'll never be happy with it afterwards.
Like you said, 77 inches of yew is too good to waste - so don't waste it by chasing a design that doesn't work. Make a stunning deflex/reflex bow that hits hard and looks perfect, and wait until you find a more suitable piece of yew for a longbow.