Piston design and spark plug location will have a lot to do with how much spark lead a motor needs.A long time ago I had a 292 ci sbc with big dome pistons and gm strait plug heads,that motor liked 50 to 52 deg.on 260 sunoco pump gas.Now I have a 412 ci sbc flat top pistons 18 deg head that likes 30 to 32 deg. with vp c12.Just some things I've seen with timing.Bill.