Heartwood is generally what you want to use with BL. Treat it just like osage but make it longer and wider.
57" is short for a 28" draw. If you build it so it bends from tip to tip, through the handle it may work out for you with good tillering. Locust it susceptable to fretting if things are not perfect and with this short of a bow, even more so.
I'm working on a 65"t/t Eastern Woodland style locust bow now. I'm taking it very slowly and exercising the wood well between wood removal sessions because I have a bad history with locust. I cut my bow building teeth on locust and it never failed to disappoint me. I'm sure it was me because I know quite a few bowyers that have successfully built BL bows. George Tsoukalas is one noted bowyer that successfully builds BL bows and has for years.