1. Look on the frame member on the front cowl between the mast and chassis for a truck serial number stamped in the frame. If the unit has been repaint several times you'll need to to some paint scrapping..
2. Are you sure the engine is a Chrysler flat head - seems to me in the 60's Yale used Continental Flat heads 4 (F163) & 6 (F227)cylinder & in the late 60's or real early 70's went to the Chrysler slant 6, H225 OHV engine in their G83C & G83P series.
3. Are the tires press on solid or pneumatic (truck tire style) shaped. The complete tire size should be on the sidewall of the tire
4. Back them the nameplates were aluminum plates pop riveted on the dash or seat deck or in the operator foot area on a vertical member - look for 4 holes that form a rectangle in this area. If you find them & no plate - it's missing. You'll need to find the s/n.
Hope this helps - being that old, parts will be hard to find anywhere.