It is my understanding it is a D Body..........check this out but IDK how correct it is:
Body Code Designations
That list is just plain bad.
First thing- D Bodies were full frame Imperials 65-66. Before that there was no body designations per-se. Chrysler went to Uni-body construction in 1960, so this 300 is a full-size unibody car.
Pre 65 Chrysler Corp did not use the body designation codes as loved by the Muscle car drones. The Body designations started getting improperly applied to pre 65 cars by the Muscle car rags when they wrote about the Max Wedge cars and called them B bodies. When Exner unveiled the "shrunken" cars- they were not popular. Unfortunately the Muscle car rag damage was done and the Drones have applied that incorrect line of thought to everything Mopar ever since...and it really doesn't apply to Exner or KT Keller era Mopars beyond broad car line categorization.
Cars could be roughly divided into groupings based on wheelbase, and therefore size- anologous to the later body designations but it's really apples and oranges as just about everything body wise is quite different. Pre 65 Mopars can best be described as follows:
106 inch Short wheelbase "compact cars"- A's- Lancer, Valiant
115 inch Mid-size-B's- Really typified by the "shrunken" Exner 62 Fury/ Polara
126 inch wheelbase Full Size-C's- DeSoto Adventurer/ Chrysler line
129 inch wheelbase full frame Full size-D's- Imperials
Jack E/NJ- Your mechanic is right to be leery about the claims on that windshield for the reasons above.