Given the choice, I'd go with the four cylinder. Say you rework the QR25, ditching the balance shafts, the close-coupled cat, the spindly conrods and economy-biased valvetrain. Up the compression ratio to around 10.5:1, and build it for an 8K redline. Probably drop displacement down to 2.2L... but it'll still deliver a healthy amount of torque...Presto, modern day "S16" Silvia.