[Mr70]

I have come across many 1969-70 L-78 Vehicles with this block as their warranty replacement.
This was the typical replacement block for the 396-402/300Hp-330Hp-350Hp & 375Hp equipped vehicles later on in their life.It replaced their original #854 blocks.
The #290 block was originally released in 1972 & casted as a 2-bolt and 4-bolt.
2-bolt for the 1972 402/240Hp Chevelle-Camaro-Passenger series,but with your #291 heads/pistons it sounds like the solid lifter 4-bolt replacement block to me.
To be sure,just look above the oil filter mounting boss for two enlarged pipe threaded holes,(3/8" & 1/2" respectively),to accept an external oil cooler if so desired.That would call out 4-bolt mains.
The CE date you relayed is a 1973 replacement vintage,the #50491 unit produced from the Tonawanda plant.
Value???..I really don't know what someones willing to pay today..the most being to an owner with a vehicle fitting this description I would imagine.
I think those rods/pistons/heads are the meat of the value.