It is funny how people get all excited over HQ SS's, and I get the significance of the cars but there were around 2800 of them made which is very common in the scheme of things and they were really only a HQ Belmont sedan with a 253, 4spd and a few other bits and pieces. I know the SS is essentially exactly what a HQ Sandman is on a ute or van but we are talking somewhere near 10% of that number and you could get a 308 in them. Outside of Statesman Deville these 350 powered HQ's are a very limited build car even where they were a normal production item. Here are the build numbers of the actual vehicles available with 350 to display what I mean:

HQ LS: 213
HQ XW8: 212 (plus a handful more after HQ Elizabeth records ceased, maybe 230 or so).
HQ 1837: 402

So these cars are sitting down less than A9X and L34 numbers for the LS and XW8, and getting close to the 150 x final spec XU-1's. They are not purpose built race cars but they are premium performance, so the top spec version of their respective models. I guess you can see why I get a bit cynical about the prices being asked for SLR5000's where there were 1000's made when you can buy any of those listed above cheaper.
Also shows how this HQ wagon would at least be extremely valueable as a parts donor to an owner of one of the 91 x LS's, 208 x 1837's or 65 x XW8's built in Dandenong that was misssing its original driveline. They aren't going to find the parts anywhere else.