When I raced the MC28 back in '95-'96 we went straight to the rear head on the front conversion without ever trying it with the front head. When i tried setting a 21 up with stock heads (but short barrels) we had all sorts of detonation problems on the front cylinder, totally cured by fitting a rear head.

Just look where the spark plug is on the stock front head and you'll realise how far the flame front has to travel to get to the front side of the head. It's just too far and detonation will strike before combustion is complete. The rear head still suffers, but by nowhere near as much.