The shop manual spells it out clearly enough that a specific tech tip seems unnecessary:
Floor to frame at front mount = 1 53/64” (1.828”) to 1 57/64” (1.890”)
Rear floor support to frame rail = 49/64” (.765”) to 53/64” (.828”)
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0102.htmIMO to mount it you need 4 friends, a pair of 4 x 4’s, and beer. You guide them into position and slip the front bolts thru and then buckle up the rear shackles. If you’re really clever you can make blocks of wood like the dimensions above to set it on and that will tell you where you are.
To add: notice the entire tolerance of .062". This means your cab should be plus or minus only 1/32" in relation to the framerail which seems pretty specific to me.
