Look at how many valve cover bolts
Five bolts - FE Big Block 352, 390, 427, 428 plus a myriad of truck engines
Six bolts - Windsor small block 221, 260, 289, 302, 351W
Seven bolts - Lima big block 429/460
Eight bots - Cleveland 351C, 351M, 400
Now, as for originality, you've got me. About all you can do is look at the casting numbers above the starter. One will be the casting number, and the other smaller number is the date the block was cast. Usually, there is a stamped serial number on the drivers side rear of the block.