Hi Andy:
As Toivo said you can identify the block by the screw-in freeze plugs.
Benefits of the Boss block in addition to four-bolt mains:
Cast of higher nodular iron, with thicker cylinder walls. Not sure why your block would have two sleeves, something to think about.
'69 Boss crank is cross-drilled steel (best). The '70 was a steel crank, but not coss-drilled (two
oil holes per throw).
Boss rods were 3/8" diameter bolts compared to 5/16" (weak point) on all other blocks.
Solid-lifter cam.
Since you might replace the internals such as pistons, rods, even crank, the benefits of this block boil down to better main cap design and stronger and beefier block.
THe cylinder heads are to die for if you can get them. They offer 2.19" intakes and 1.71" exhaust valves (big as big block).
Bill