Doel-Fin/stabelizer where to position it on lower unit?

[Marshall regal]

Q. Have a Regal, 17 footer, merc io, and want to put on a stabilizer /Hydrofoil on the lower unit..Its not large, maybe 6-8 inch span, Doel-Fin do I bore the holes before the prop, or on top of it near the end of the lower unit? Dont want to damage the hydrolic or lower unit..just need to know the best place to position it...Im thinking right below where the lower unit meets the upper neck.... is this correct? thanks.