Mike,
First off, does the car have SS badges? Is the tailpan
painted black? Does the car have multi-leaf springs? If
so, are they 4 or 5 leaf versions? Do the heater hoses
enter the heater core cover ( @ firewall ) in the center,
or at the end closest to the engine? Does the radiator
have 3 or 4 cores? Is the upper radiator outlet straight,
or bent 90 degrees ( toward the passenger side )? Does the
THM400 have a flat stamped steel cross-member, or tubular?
My 69 L/78 was born 10-D-68 also. It was originally a
THM400 car as well. Post what you find. It will start
the process of determining if it was a small or big block
originally.
Steve