doubt you would fit an RB without cutting the firewall, the take up a bit more room length ways in a silvia than an SR does. SR having the rather large fan on the front of it and a big fan shroud...room which the RB takes up

but yes it does look like hes gone a fair bit further back than what would be needed just to fit the engine in, as its got more room between the engine and radiator than the CA does in my car