They take your existing RB26 and rebuild it from bottom up.
For a quick run down:
Block / Head machined to Racepace specs / Strengthen / Blueprint Block similar to N1 specs.
Block is bored, honed, decked, crank tunnel line honed to Racepace specs.
Modified oil / tap / water galleries, fit threaded plugs / welsh plugs.
Pistons / Piston Rings / Conrods / Crankshaft are a mixture of OEM (Racepace Modified) / N1 / Nismo / Custom.
They use a Nissan N1 Oil Pump (Modified) even though a lot of people talk shit about N1 Oil Pumps, Racepace make it work.
Cylinder Head: Replace valve guides / stem seals, cut seats, face valves, match chambers, surface, re-shim valve clearances to RPM specs
Modified Combustion Chamber Shape.
Labour is included.