Help with my first engine build (5.9 Magnum)

Magnums used the 360 pan. A 273-318-340 pan won't work. This should Mancini Racing Center Sump Oil Pan Kit IIRC the conversion gasket is for using the one piece Magnum gasket with the LA360 timing cover and pan as opposed to the 4-piece LA gasket. If you plan to run a fuel pump you are most likely using a LA timing cover so you should be able to just use a LA360 pan gasket set.