If you're looking at total numbers produced rather than individual years, the BOSS 429 would be the rarest, followed by BOSS 302, then Shelby GT350. All are very different cars though. BOSS cars, Shelbys are individual models rather than engine code options. R codes and K codes are more comparable. Over 3 years there were approx 18,000 R codes against approx 13,000 K codes.
I think the best way to look at it is comparing K codes to other Mustang engine options of the particular year produced.
67 K codes are one of the rarest of all! - but I'm biased.
Edited by - Rohan on 01/02/2009 15:51:52