Mike, the transmissions were not exactly the same for cars and trucks. The trans cases on cars were drilled for 4 bolts to be installed into the bell housing from the trans side. On the trucks the lower mount holes were threaded on the transmission side whereby the bolts were installed from inside the bellhousing. The side covers are different and 58 up rear housings also had a pad for a rear mount even if the mount was not used on trucks.
From what I have seen with regard to date codes, the later date code would be installed in the next year - for instance a May date code would likely be installed in a 57 and a July or Aug date code would likely be installed in a 58.
With all the mixing and matching that has taken place over the last 50 or 60 years, it would be extremely hard to prove what the vehicle had in it originally unless someone had a completely original vehicle that hadn't ever been touched.
To me, a trans with a truck case, rear mount, and a late 57 or 58 date code would have likely been installed in a 58 truck.
Fred
