Can you check the thread count/measurement. Chevy used very few bolts in the engine area that were around 3-1/2" long from what I can find in the parts books. I can find bolts listed in the 1929-46 Chevy Master Parts Book as 7/16-20 x 3-1/2" and a 1/2-13 x 3-1/2 in the general hardware list, but neither match engine mount bolts for your truck.
For your truck, there is an engine mount bolt listed as "Bolt, rear hold down (7/16-20 x 3-3/4 drld.) GM# 594121 with a requirement of two (2) bolts, but that sounds different than what you have pictured.
There is the possibility your existing bolts were used to replace other similarly sized bolts.
