I don't have any documentation on that. Tonawanda Engine records for 65 and later should show it if they were still being cast.
I've heard anecdotal stories that they were still available through parts department through sometime in early 70's but have no documentation there, either, and even if they were, they may have been slow moving inventory of new old stock instead of continued production. Perhaps a check of truck parts catalogs for those years would verify that they were still available beyond the date of my newest Truck Parts Catalog which is May. 1966 and partial engines, fitted blocks and heads were still listed.
As far as the question of whether the W was orignally designed for truck only and added to the car line as an afterthought, I think the article written by Don Francisco for Hot Rodding Magazine pretty much covers it in this excerpt:
"This may not seem to make much sense, but it was the immediate success of Chevy's 265-inch V8 when it was introduced in 1955 that prompted development of the 348. Demands for the 265 by many thousands of motorists to whom it brought their first taste of enjoyable driving presented Chevrolet with the problem of producing the engines fast enough to meet the orders. The only answer to the problem was additional engine production facilities. As such facilities are expensive, it was decided after a long look at the problem to design them for an engine that would be suitable for both passenger and trucks being produced then and in the forseeable future. This would make them good for many years. When it was determined that the 265-inch engine did not have the displacement potential it was estimated would soon be necessay, and that cylinder head production for it to meet different compression ratio demands had become too involved, the route was clear: design a new engine.
A set of design requirements was drawn up for the new engine. It woud have to be adaptable to a broad range of displacement with a minimum number of changes to its parts. It would have to be adaptable to a broad range of compression ratios to enable full advantage to be taken of future fuels. It would have to have exterior dimensions suitable for present and future passenger car installations. It would have to have mounting provisions for both passenger car and truck accessories. It would have to be easily modified for future demands with existing machine tools."
I think it is unrealistic and illogical to believe that the W was designed for trucks and at the last minute they decided to put it into the passenger car line. GM, as did every other manufacturer, could not afford to do things on a whim. Everthing was planned and engineered years in advance. They knew that cars were getting bigger and heavier and they would need a powerplant with the torque and horsepower to propell them down the road with a reasonable power to weight ratio to do so without overloading the smaller engines. Yes, they still installed 6 cyl and small block engines in the big cars for years after, but most of the 348's went into the heavier Impalas and Bel Airs with lots of accessories.
In short, the engine was purposely designed to meet multiple market needs in the most cost effective way.
No way was it designed specifically for trucks.
That's my story and I'm sticking to it.