Full dirt: https://1969pontiac.blogspot.com/2017/11/origins-of-grail-corvette-powered.html

Short version:

I found a 1969 Pontiac 427/390 Four Speed M21 Convertible in a barn in Northern Canada. You might be asking "Why do we care", and you might not, but Canadian Pontiacs are really Impalas underneath in that era, and the 2+2 was really an Impala SS underneath.

I've got the car blown completely apart at the moment, with the chassis off for powder and the L36 off at the machine shop.

Mechanically the only big surprise I've run into is the 3-core rad and 4-blade fan. If you got the L36 (or maybe even L72 in the US) you only got the small rad and fan unless you optioned up to HD Cooling or AC.