It's probably economics as much as anything. A GMC has always been more costly to rebuild than a 235 and performance parts are available only through a couple of aftermarket niche suppliers. $600 for Venolia pistons are a bit steep for most folks. Building a 302 Jimmy with all the right pieces can eat up as much money as big block V8.
Plus you almost need to have another GMC core engine to use for the oil pan, flywheel etc which are too massive to use as is.
As you mention, the 302 GMC engine is not familiar to most parts sources - it was a heavy truck motor only, in production for a relatively brief period (1952-1959). Not hard to find? Well maybe not in your beautiful state of South Dakota, but old junkyards are disappearing fast in many parts of the country. Finding one with a head that's not cracked can be a challenge too.
The 302 is a great motor and can make some big torque, but for most Stovebolters it's probably hard to justify the cost and extra work.
