PT = Trenton Plant
383 = 383cubic inches
P = Premium fuel
2351 = assembled Thurs, 4 Jan 1968
74? = number assembled on that date

Pretty sure but not absolute about the "P".

Being a '68 the VIN should be stamped on top of the rear of engine where bellhousing/auto trans attaches near oil sending unit. But that could be hit or miss.

There should be some stamping on the block mating surface just in fron of where the passenger side head attaches.

Frank

Remember - 2 wrongs don't make a right - but 3 rights make a left!