Conver fishing boat to gaff rigting 50ft

[felicity]

I'm converting a wooden 50ft fishing boat to gaff rig. I need to know where to put it and how to construct/what length. Keel stepped would I fel be better.

[alan white]

The work of converting begins on the drawing board, including whether it is even feasible. If you don't know how to arrange the sailplan, you probably didn't know whether the conversion could really be done and make sense.
Most likely, it can't be sensibly done.
To place the sails, you need to produce a hull design or an accurate drawing of the lines. It must include the keel and the rudder.
It's not impossible to do what you want to do. But even under the best of circumstances (the hull lends itself somewhat to sailing, etc.), the stresses of a sailing rig are entirely unanticipated by the existing design, meaning the support of rigging loads, mast compression, etc., not to mention you'd need to lower the tophamper/windage and provide adaquate ballast down low,