Possibly. By today's surviving cars you'd have to say a HQ 6cyl Sandman (combined across the 3 x 6cyl models available) could be the rarest. HQ XW8 (GTS350 sedan) were pretty rare with somewhere between 212 and 230 or thereabouts built. I know this is a V8 Kingswood, or less generically a GTS sedan and there were lots of 253 and 308 examples, but this was supposed to be a separate vehicle like XX7, or XU1 etc, and not meant to be counted or classed as its base vehicle. GTS350 coupe was probably more common than Sandman with 402. LS coupe with 350 is pretty rare too at 213 but there were plenty more 202, 253 and 308 examples. If you don't count Sandmans, V8 HQ vans would be pretty scarce (6cyl and V8 vans were separate models). V8 HQ cab/chassis would be rare too, these weren't available until around 3/73.