Maybe the 10Bolt's been swapped in but the L36/M40 combo was available also the L72/425Hp too!.
Is it here in Canada also have you checked the build plant digit of the Vin?.
Besides being available for cars sold new in Canada, GM Vintage Services info packages are available for vehicles shipped new across the US/Canada border so lots of US delivered cars built at Canadian plants are included too!.
Click below..
http://www.vintagevehicleservices.com/options.html
[img]<<GRAEMLIN_URL>>/beers.gif[/img]
~ Pete