As I recall, the B engines were all big blocks starting with the 350 and the 361 in 1958 (350/361/383/400). The LA engines that came along afterwards starting with the 273 in late 1964/early 65 were all small blocks (273/318/340/360). Incidentally, the old A engines that they introduced back in 1956 or so were the ones that morphed into the LA lineup. LA actually stood for "Light A" because they were so much lighter in weight.
I was a Chevy man and I ran against too many of these motors to not know them!! When money changed hands I wanted to know what I was up against! Ahhh, the good ole days!!