[davepl]

I don't know about the 305 per se, but on the rest of the motors they did NOT stamp the partial VIN or sequence. So my original motor 396 truck can only be documented by dates and castings.

That's likely the best you'll do as well - the right 3-letter casting on the motor to say it's for a 67 truck, and the truck saying it came with a 305, and the dates on the motor preceding the date of the truck build by some reasonable margin.