I'd say go with the sr20, If mitsubishi can run 6.8's with their 4g63 and nissan can run 7.2 with their sr20. 2jz's and rb's have trouble getting In that range with a frame car and not a rail car.
Shit 13b rotary's run 6.8's.
YouTube - RX7 6 second drag run
YouTube - 6 second Mitsubishi Eclipse
YouTube - Import DPS Nissan 240SX Ka-T 7.87 @ 174MPH
^^^ all that 2.4 and can't even run faster than the sr's or 4g63's. Like I've always said Ka's are nice but just not what people think they are.
YouTube - Brent rau vs. mazworx s15