I've had S30's with small block as well as rotary's but they just don't sound right for the car. I started a RB25 swap in my 72 S30 but with other projects that is on a back burner at the moment. I don't know if you had a chance to check out the YouTube I posted but nothing beats the sound of triple Webber's screaming... who needs a stereo:)
Well rust was the biggest problem with 60-70's cars from all corners with the exception of German.