I would say you are in for an uphill fight making English Wych Elm that heavy, its possible, anything is but this will really test you. If you get everything bending nice early on and the wood is very good, then heat treat it totally before it takes any set. If you make the bow 78"be prepared to take it down to 76-75" to get the weight up. There are a few in the EWBS that are making those weights with white woods, it may be worth joining.