grout filling has no cooling issues as the top of the bores pistons is where all the heat is. Tk from PForums an experienced well known builder fills most of the high HP holden v8s he builds with over 500hp for the street and track and mostly all have cast iron heads with no cooling issues.

81mm is a breeze for the engine shop for an A14 with A15 crank rods and vw slugs. The cement filling is not really cement but a 2 pack polymer that matches the cast iron properties for heat dissipation and other factors.