Ditto-no such critter as a 4 bolt main 283. Only the 350, early 400, and '68-'69 302 ahd 4-bolt mains. There is a difference in the block casting for 4-bolt engines, the webs between the cylinders are thicker. Not all engines with the thicker webs are 4-bolt, however. They did build 2-bolt versions of those blocks, mainly for light truck use. They can be machined to accept them, though.
