Casting numbers won't do much good because the direct and OD are built in the same case. 50 years ago it was common to change a few gears in a Clark and make an OD out of it. GMC would have a line set inside the glove box door. I don't know much about Chev. If all else fails put it where 4th would be on a direct.
2 4 R
|_|_| Direct Drive 5th
| | |
3 5 1
Turn the input shaft. If the output goes slower than the input, you have a direct. If it goes faster, you have an overdrive.
