This may help clear up our information on this thread and also the thread on the 307 engine.

307 engine---- In 1968 this engine was introduced to fill the void left by the 283 engine. Using the then new large journal crank, it retains the bore of the 283ci block,but uses the 3.25 stroke of the 327 engine. It is not a particularly desirable block,as most of them were cast with very little nickel and no tin to improve the cast mix. This was an attempt to lower the production cost of the bottom line engine. It seems to have an increased bore wear rate
because of that.
I took this word for word out of the Motorbooks International ,
Chevrolet small block V-8 ID guidebook.

It also states in its Engine Blocks section------
When looking for a block with a high-nickel, high tin mix, the numbers to look for are "010" and "020". They are cast under the
timing cover area. They show a picture of a block with both those numbers cast there.
My 4-bolt main 350 motor has both those numbers cast there.
As we discussed earlier you can make a 350 or a 327 out of a 307
but ,this shows us that it is not ideal . As I suggested earlier, this may work fine for a good little street motor ,but for any all out performance----Don't go there.

Malibu Chevelle, I'd still like to know your source of information,as I can't find anything like what you suggest. It may be so but I just cant find it anywhere.

The most desirableble 350 blocks were the early 70s blocks called the K series. They have a K prefix on the serial # and a K before the casting # and a K on the main caps. By 1978 the blocks were lighter and not as strong.
Any block made in Mexico is stamped "hecho en Mexico" Many of these were sold over the counter at you local Gm parts dept. Hope this helps. Gene