give examples then as to how I am wrong in order to help the thread poster

identical parts between 2wd and RS500 engine are:-

Pistons
Rods
Crankshaft
Front Crank Cover
Head Gasket
Big ends and Mains
Bearing Caps
Valves
Cams
Cam Caps
Hydraulic tappets
Combustion Chamber volume
Cam Cover
Cambelt
Cambelt cover
Injectors
Coil
Leads
Inlet trumpets - Rs500 version was slightly machined at the mouth about 5mm in after casting and had two dowl holes drilled into it but casting is the same
Exhaust Manifold
Flywheel and clutch
Crank Pulley
Sump
Distributor
Engine arms and mounts
Modine
Breather set up
Idle speed control valve
throttle position sensor
crank sensor
all pulleys
map sensor
amal valve

The only different parts were :-

block - but internally the same - and could be swapped for a normal 205 and still work fine at normal outputs
turbo
Plenum outer body to accept further 4 injectors and throttle body
Upper fuel rail to allow extra take off point to the extra injectors
Fuel regulator - to allow twin inlets
3 bolt mounting point for the revised thermostat housing - other than that the casting is as per 2wd
water pump
bracket for the revised location of the amal valve
turbo damper
and a few minor odds and ends like air filter top, L1 ecu slightly modded etc

All first hand fact not pub talk - I have personally stripped and rebuilt every version of the YB engine inc RS500, only one I have still to play with is an EECV late escort spec one

To give you an idea of similarity you could take out the engine with all bolt ons fitted from a 3dr Cosworth and an RS500 swap them over refit them in the different shells - plug the looms back into the connectors and they would fire up and run fine in the wrong car

I am a Consultant Quantity Surveyor with no business connection to Cosworths at all so lets see if the "experts" know more than me