Forget the GMC manual, what does your 1936 Chevrolet Repair Manual say? As you mentioned previously you tried the motor mount on top of the cross member and the engine sat too high.
According to the 1936 Chevrolet parts book the front motor mount is the same for 1932 passenger cars and 1933-36 trucks. The motor mount for 1932 mounts UNDER the front cross member....it does not sit on top. Don't let the shield confuse you. Try the motor mount under the cross member and see how your engine sits.
Taken from GMC maintainence Manual, Section 8, page 175. The GMC engine configuration is the same as the chevy.
Information on the Internet regarding the 1936 GMC engine indicates the following:
1934-1937 - Oldsmobile 6-cylinder L-head engines were used in GMC light and some medium duty trucks.
1936 - Major changes appeared in all GMC conventional cab models. 15 new models were released, including 1/2-ton pickup and panel trucks with Olds engines. All other models had GMC engines that were in nine sizes from 239 to 707 cid.
