As you have a Poly, I will assume a 66-older trans, which has a different spline count than 67-newer, which a 360 converter would require.
If you get a 360 converter, you can knock the weights off and weld onto teh flexplate or the Poly converter - just pay attention and get then in located in a similar location.
I heard the 360 had a different motormount on one side from the 318, but that some washers or spacers can solve that? IIRC.
If this is going in a 65 C-body - the driverside motormount is 1-year-only, and might require additional fiddling to moutn a 360.
Those are probably the big compatibility things - everything else is the standard connections and linkage, etc, involved with any engine swap.