The '68 part is probably from casting dates?.
Is it a 512 block...and is there a distrib there too, & the #'s?.
Can you lay all the stamped info on us?.
The closest BB suffix to be confused could be the MH which was an L36 Vette motor...I believe all other MN's were SB?.
As far as the $ value question goes, ChucKing probably has a better idea than anyone on what these actually will reach?.
Important is retrieving and providing the Vin as mentioned above as this could prove interesting also have an impact on the value?.
~ Pete